-
MAGDALENA TODA:
We have any people
-
who finished the
extra credit and are
-
willing to give it to me today?
-
I mean, you don't have to.
-
That's why it's
called extra credit.
-
But I think it's good
for extra practice
-
and for the extra points.
-
So hold on to it if you cannot
give it to me right now.
-
And I'll collect it at
the end of the class.
-
Today's a big day.
-
We are starting a new
chapter, Chapter 11.
-
-
So practically, we are
going to discuss all
-
through this chapter functions
of several variables.
-
-
And you are going to
ask me, wait a minute,
-
why do we need functions
in more than one variable?
-
Well, we are all functions
of many variables.
-
I was freezing outside,
and I was thinking,
-
I'm a function of
everything I eat.
-
I'm a function of the
temperature outside.
-
Almost everything in
our body is a function
-
of hundreds of factors,
actually, thousands.
-
But we don't have the time
and the precise information
-
to analyze all the
parameters that
-
affect our physical
condition every day.
-
We are getting there.
-
I'm going to give you
just the simple case.
-
So instead of y equals f of x
type of function, one variable,
-
we are going to look at
functions of the types z
-
equals f of xy.
-
-
Can I have many more?
-
Absolutely I can.
-
And that's kind of
the idea, that I
-
can have a function
in an-- let me
-
count-- n plus 1
dimensional space
-
as being of the type xn plus
1 equals f of x1, x2, x3, x4.
-
Somebody stop me. xn.
-
Right.
-
I have many variables.
-
And that is a problem
that affects everything.
-
Our physical world is
affected by many parameters.
-
-
In engineering
problems, you've already
-
seen some of these parameters.
-
Can you give me some
examples of parameters you've
-
seen in engineering classes?
-
x1, x2, x3 could be the
Euclidean coordinates, right,
-
for the three [? space. ?] But
besides those, there was an x4.
-
It could be?
-
Time.
-
Excellent, [INAUDIBLE].
-
More than that.
-
I want more.
-
I want x5.
-
Who can think of
another parameter
-
that affects physical processes
or chemical reactions?
-
Yes, sir?
-
STUDENT: Temperature.
-
MAGDALENA TODA: Temperature.
-
Excellent.
-
Another very good idea.
-
How about x6?
-
I'm running out of imagination.
-
But you have a lot more
information than me.
-
Pressure.
-
Maybe I'm studying a process of
somewhere up in the atmosphere.
-
Maybe I'm in an
airplane, and then it
-
becomes a little bit
more complicated,
-
because I hate the way
cabins are pressurized.
-
I can feel very uneasy.
-
My ears pop and so on.
-
We can be in the
bottom of the ocean.
-
There are very many
physical parameters
-
that affect physical
processes, chemical processes,
-
biological processes.
-
I don't know if this is
fortunate or unfortunate,
-
but I think that was
the key to the existence
-
of the universe in the first
place-- all these parameters.
-
OK.
-
Let me give you a simple
example of a function that
-
looks like a graph.
-
This is a graph.
-
-
And you say, wait a
minute, wait a minute.
-
Can I have functions of several
variables that cannot be
-
represented as graphs?
-
Yeah.
-
Absolutely.
-
We will talk about that
a little bit later.
-
So if I were to give you
an example that you've
-
seen before, and I would say,
give me a good approximation
-
to a valley that is actually
a quadric that we love and we
-
studied before for
the first time.
-
That quadric is a
beautiful object, a valley.
-
Any imagination,
recognition, recollection?
-
I know I scared
you enough for you
-
to know the equations of those
quadrics since some of you
-
told me we watched
all the videos,
-
we read all the stinking
book like never before.
-
That was kind of the idea.
-
I didn't want to scare you away.
-
I wanted to scare you
enough to read the book
-
and watch the videos.
-
And I'm talking about a valley
that you've seen before.
-
Many of you told me you like
the University of Minnesota
-
website that has the
quadric gallery of quadrics.
-
-
So you've met this guy before.
-
They show the general equation.
-
But I said I like the
circular paraboloid.
-
So they talk about
elliptic paraboloid.
-
Which one do you think I prefer?
-
The circular paraboloid.
-
Give me an example of
a circular paraboloid.
-
STUDENT: A flashlight?
-
Inside.
-
MAGDALENA TODA: The expression,
the mathematical equation.
-
STUDENT: Oh, sorry.
-
So it would be x
squred plus y squared.
-
MAGDALENA TODA: Very good.
-
That's exactly
what I had in mind.
-
Of course, it could be
over something, over r.
-
All right.
-
That's my favorite.
-
Now, if I put the flashlight
in here just like one of you
-
said, or the sign on
top of the z-axis.
-
Then I'm going to look at
the various-- we discussed
-
that a little bit before.
-
So various horizontal
planes, they're going to cut.
-
They're going to cut the
surface in different circles,
-
upon different circles.
-
We love them, and we use them.
-
And what did we do
with them last time?
-
We projected them on the floor.
-
And by floor, I mean the what?
-
By floor, I mean the xy plane.
-
Plus this xy plane.
-
I label it like you like it.
-
You said you like
it when I label it,
-
so you have the
imagination of a table.
-
This is x and y and z.
-
And so I gave you an example
of a graph cut in with z equals
-
constant positive or negative?
-
Well, it better be positive,
because for negative, I
-
have no solutions.
-
Positive or zero.
-
Well, for zero I have
a degenerate conic.
-
A degenerate conic
could be a point,
-
or it could be a bunch of lines.
-
In this case, all those
circles-- doo-doo-doo-doo-doo--
-
a family of one parameter,
family of circles.
-
Like the ones that
is-- a dolphin
-
is now doing that
in San Antonio,
-
San Diego-- to take
those old circles
-
from the bottom of the sea,
and bring them different sizes,
-
and put them together.
-
So they are very smart.
-
I love dolphins.
-
So we'll see 0
[INAUDIBLE] get a point.
-
That's still a conic.
-
It's a degenerate circle.
-
Do you realize
that's a limit case?
-
It's really beautiful.
-
You know what I mean?
-
Circle on top of a circle
on top of a circle,
-
smaller and smaller.
-
All right.
-
So good.
-
If I create shadows--
because that's
-
why you guys wanted
the source of light
-
on top-- of the projections
of these circles,
-
I'm going to have them
at the same color.
-
But dotted lines because I
think the book doesn't show them
-
dotted.
-
But on my way here,
I was thinking,
-
I think it's more beautiful
if I draw them dotted.
-
And how big is this circle?
-
Well, god knows.
-
I'm going to make a purple
circle that is, of course,
-
equal in size, equal in radius
with the original purple
-
circle.
-
So the dotted purple circle,
that's on the ground--
-
is just the projection of
the continuous purple circle.
-
It's identical in radius.
-
So for the family of
circles on the surface,
-
I have a family of projections
on the ground in the xy plane.
-
And such a family of
projections represents
-
a bunch of level curves.
-
We call this family
of level curves.
-
-
OK?
-
All right.
-
So if you think about it,
what are level curves?
-
You view them as being in plane.
-
Oh, my god.
-
So I should view them as a bunch
of points, a set of points.
-
If I make it like
that, that means
-
I view this as an
element of what?
-
Element of the xy plane,
right, with the property
-
that f of x and y is a constant.
-
-
OK?
-
In my case, I have a
[INAUDIBLE] constant.
-
In general, I have an
arbitrary real constant.
-
That's a level curve for
level C, for the level
-
C called the level, or altitude
would be the same thing.
-
So have you seen these
guys in geography?
-
What in the world are these
level curves in geography?
-
STUDENT: [INAUDIBLE]
show the slope
-
of a-- the steepness of a hill.
-
MAGDALENA TODA: You've
seen topographical maps.
-
And I'm going to try
and draw one of them.
-
I don't know, guys,
how-- excuse me.
-
I'm not very good
today at drawing.
-
But I'll do my best.
-
It could be a temperature
map or pressure map.
-
[INAUDIBLE] or whatever.
-
Now I'll say, this is
going to go-- well,
-
I cannot draw the
infinite family.
-
I have a one-parameter family.
-
And then I'll-- I'm dreaming of
the sea, summer break already.
-
You see what I'm doing.
-
Do you know what I'm doing?
-
That means I'm dreaming of the
different depths of the sea.
-
So for every such broad
line, I have the same depth.
-
The same altitude for
every continuous rule.
-
The same depth
for every-- so OK.
-
I'm not going to swim
too far, because that's
-
where the sharks are.
-
And I cannot draw the sharks,
but I ask you to imagine them.
-
It's fundamental in
a calculus class.
-
So somewhere here
I'm going to have--
-
what's the deepest--
guys, what's
-
the deepest point in that?
-
[? STUDENT: 11,300. ?]
-
MAGDALENA TODA: And
do you know the name?
-
I know the--
-
STUDENT: Mariana Trench.
-
MAGDALENA TODA: Mariana Trench.
-
STUDENT: Trench.
-
MAGDALENA TODA: All right.
-
So these topographical
are full of curves.
-
These are level curves.
-
So you didn't know,
but there is a lot
-
of mathematics in geography.
-
And there is a
lot of mathematics
-
in-- oh, you knew it.
-
When you watch the
weather report,
-
that's all mathematics, right?
-
It shows you the distribution
of temperatures everyday.
-
That is what we can
[INAUDIBLE] also
-
care about other functions
of several parameters, right?
-
And those functions could
be pressure, wind, whatever.
-
OK.
-
Speed of the wind.
-
Something like that.
-
I did not dare to
look at the prediction
-
of the weather for this place.
-
This place used to
be a beautiful place.
-
300 days of the
year of sunshine.
-
Not anymore.
-
So there is something
fishy in Denmark
-
and also something
fishy in [INAUDIBLE].
-
The world is changing.
-
So if you don't believe in
global warming, think again,
-
and global cooling, think again.
-
All right.
-
So unfortunately,
I am afraid still
-
to look at the temperatures
for the next few days.
-
But--
-
STUDENT: It's going to
be 80 degrees on Tuesday.
-
MAGDALENA TODA: Really? [?
-
Well, see, I should
have looked at it. ?]
-
[LAUGHTER]
-
I should gather the
courage, because I
-
knew-- when I was
interviewed here
-
for assistant professor,
gosh, I was young.
-
2001.
-
And my interview
was in mid-February.
-
And birds were chirping, it was
blue skies, beautiful flowers
-
everywhere on campus.
-
And I love the campus.
-
OK.
-
Give me an example of a surface
that cannot be represented
-
as a graph in its
entirety as a whole graph.
-
You gave me that before,
and I was so proud of you.
-
It was a--
-
-
[LAUGHS]
-
-
What kind of surface
am I trying to mimic?
-
STUDENT: A saddle.
-
-
MAGDALENA TODA: That
can be actually a graph.
-
That's a good
example of a graph.
-
A saddle.
-
But give me an example
of a non-graph that
-
is given as an implicit form.
-
So graph or explicit
is the same thing.
-
z equals f of xy.
-
Give me a non-graph.
-
One of you said it.
-
x squared plus y squared
plus z squared equals 1.
-
Why is this not a graph?
-
Not a graph.
-
Why is this not a graph?
-
-
STUDENT: [INAUDIBLE].
-
When you move it over to
1, you can't actually--
-
MAGDALENA TODA: You
cannot but you can cut it.
-
You can take a
sword and-- I'm OK.
-
I don't want to think about it.
-
So z is going to be two graphs.
-
So I can split this surface
even in a parametric form
-
as two different graphs.
-
Different graphs.
-
If I cut along-- I have this
orange, or sphere, globe.
-
And I cut it along
a great circle.
-
It doesn't have
to be the equator.
-
But you have to
imagine something
-
like the world and the equator.
-
This is kind of in
the unit sphere.
-
Today I drank enough
coffee to draw better.
-
Why don't I draw better?
-
I have no idea.
-
So that's not bad, though.
-
OK.
-
So that's the unit sphere.
-
What does it mean?
-
It means it has radius how much?
-
STUDENT: 1.
-
MAGDALENA TODA: 1.
-
Radius 1, and we
are happy about it.
-
And it has two graphs.
-
It's not one graph,
it's two graphs.
-
So this is called
implicit equation.
-
This is your lab
from-- I was chatting
-
with-- instead of
studying last night,
-
I was chatting with
you at midnight.
-
And one of you said, if I had
something I hated in calculus,
-
it was the implicit
differentiation.
-
And I know this is
your weak point.
-
So we'll do a lot of
implicit differentiation,
-
so you become more comfortable.
-
Usually we have one exercise in
this differentiation at least
-
on the final.
-
So this is an implicit equation.
-
And z is going to be
two graphs-- 1 minus x
-
squared minus y squared.
-
So I have, like, two charts,
two different charts.
-
OK.
-
-
The upper hemisphere--
I'm talking geography,
-
but that's how we talk
in geometry as well.
-
So geography right
now is like geometry.
-
I have a north pole.
-
Somebody quickly give me the
coordinates of the north pole.
-
STUDENT: 0, 0, 1.
-
MAGDALENA TODA: 0, 0, 1.
-
Thank you, Brian.
-
0, 0, 1.
-
How about the south pole?
-
STUDENT: 0, 0, minus 1.
-
MAGDALENA TODA: 0, 0, minus 1.
-
And write yourself a
note, because as you know,
-
I'm very absent-minded
and I forget
-
what I eat for lunch and so on.
-
Remind me to talk
to you sometime
-
at the end of the chapter
about stereographic projection.
-
It's a very important
mathematical notion
-
that also has to do a
little bit with geography.
-
But it's a one-to-one
correspondence
-
between a certain
part of a sphere
-
and a certain huge
part of a plane.
-
Now, we're not going
to talk about that now,
-
because that's not [INAUDIBLE].
-
That's a little bit
harder [INAUDIBLE].
-
You guys should now
see this line, right?
-
This should be beyond--
in the twilight zone,
-
behind the sphere.
-
OK?
-
So you don't see it.
-
And who is this? z equals 0.
-
And so this green
fellow should be
-
the circle x squared
plus y squared equals 1
-
in the xy plane.
-
-
Good.
-
So I have two graphs.
-
Now, if I were to ask
you, what is the domain
-
and the range of the function?
-
I'm going to erase
the whole thing.
-
What is the domain and the range
of the related function, z,
-
which gives the
upper hemisphere?
-
Upper hemisphere.
-
It's a graph.
-
And square root of 1 minus
x squared minus y squared.
-
You may stare at
it until tomorrow.
-
It's not hard to figure
out what I mean by domain
-
and range of such a function.
-
You are familiar
with domain and range
-
for a function of one variable.
-
For most of you,
that's a piece of cake.
-
That was even
pre-calc wasn't it?
-
It was in Calc 1.
-
So most of you had
algebra and pre-calc.
-
Now, what is the domain
of such a function?
-
Domain of definition has to be a
set of points, x and y in plane
-
for which the
function is defined.
-
If the function is
impossible to be defined
-
for a certain pair, x, y,
you kick that couple out
-
and you say, never come back.
-
Right?
-
So what I mean by domain is
those couples that we hate.
-
Who we hate?
-
The couples x, y for which x
squared plus y squared is how?
-
-
What existence condition do I--
-
STUDENT: [INAUDIBLE].
-
MAGDALENA TODA: Yeah.
-
You see this guy
under the square root
-
has to be positive or 0.
-
Right?
-
Otherwise, there is no
square root in real numbers.
-
That's going to be
in imaginary numbers,
-
and you can take
a walk, because we
-
are in real calculus
in real time as well.
-
So x squared plus y
squared must be how?
-
Less than or equal to 1.
-
We call that a certain name.
-
This is called a
closed unit disk.
-
Please remember, I'm teaching
you a little bit more
-
than a regular Calc 3 class.
-
They will never
make a distinction.
-
What's closing with this?
-
What's opening with this?
-
Everything will come
into place when you
-
move on to advanced calculus.
-
If I don't take the boundary--
so everything inside the disk
-
except for the boundary, I have
to put strictly less than 1.
-
That's called open unit disk.
-
For advanced calculus,
this is [INAUDIBLE].
-
All right.
-
This is just a parentheses.
-
My domain is the closed one.
-
What is the range?
-
The range is going to be--
-
STUDENT: [INAUDIBLE].
-
MAGDALENA TODA: The altitude
starts having values from--
-
STUDENT: Negative 1 to 1.
-
STUDENT: 0 to 1.
-
MAGDALENA TODA: So I'm 0 to 1.
-
I'll only talk about
the upper hemisphere.
-
I should even erase,
because I don't want it.
-
So say it again, guys.
-
STUDENT: 0 to 1.
-
MAGDALENA TODA: 0.
-
Open or closed?
-
STUDENT: Open.
-
STUDENT: Closed.
-
STUDENT: Closed, closed.
-
MAGDALENA TODA: Closed to?
-
STUDENT: 1 closed.
-
MAGDALENA TODA: 1 closed.
-
Yes.
-
Because that is the north pole.
-
I've been meaning to
give you this example.
-
And give me the other example
for the lower hemisphere.
-
What's different?
-
The same domain?
-
STUDENT: It ranges from--
-
STUDENT: Negative 1.
-
STUDENT: Negative 1 to 0.
-
MAGDALENA TODA: Closed
internal, right?
-
When we include the endpoints,
we call that closed interval.
-
It has a certain
topological sense.
-
You haven't taken
topology, but very soon,
-
if you are a math major, or
you are a double major, or some
-
of you even-- they want to
learn more about topology,
-
you will learn what an open
set is versus a closed set.
-
Remember we called this closed.
-
This is open.
-
And if it's closed here and
open there, it's neither.
-
OK?
-
Don't say anything about that.
-
OK.
-
To be closed, it has to be
containing both endpoints.
-
I'm going to erase this.
-
-
And this was, of course, 11.1.
-
We are in the middle of it.
-
In 11.1, one of you gave me a
beautiful graph to think about.
-
And I'm going to give
you something to do,
-
because I don't want
you to get lazy.
-
I'm very happy you came
up with the saddle.
-
-
All right.
-
We drew such a saddle.
-
-
And I did my best,
but it's not hard.
-
It's not easy to draw saddle.
-
When I am looking at the
coordinates, x, y, z,
-
I have z equals minus y
squared will look down.
-
-
Maybe I made it too fat.
-
I'm really sorry.
-
And down.
-
This continues.
-
-
OK?
-
And then what other thing
did I want to point out?
-
I want to point out--
do you see this?
-
This should look a
little bit more round.
-
It doesn't look
round enough here.
-
STUDENT: Your'e drawing
a saddle, right?
-
MAGDALENA TODA: No, I'm
drawing just the section
-
z equals minus y squared.
-
So I took x to be 0.
-
And the purple line
should be on this wall.
-
I know you guys have
enough imagination.
-
So this is going to
be z equals minus y
-
squared drawn on yz wall.
-
-
I've done this before,
but I'm just reviewing.
-
What if it's y0?
-
Then I have to
draw on that wall.
-
And I have to draw beautifully,
which I am not-- don't always--
-
I can't always do.
-
But I'll try.
-
I have z equals x squared
drawn on that wall.
-
If I start drawing,
I'll get fired.
-
That I have this branch.
-
I should go through that
corner and go out of the room
-
and continue with that branch.
-
All right?
-
-
This is curved like
that in this direction.
-
And this other is
curved like this.
-
So if the guy is
going to put his feet,
-
where is the butt of
the writer going to sit?
-
He is here.
-
And these are his legs.
-
-
And these are his cowboy boots.
-
OK.
-
Do they look like cowboy boots?
-
No, I apologize.
-
STUDENT: Looks like socks.
-
MAGDALENA TODA: Yeah.
-
They look more like
Christmas socks.
-
But anyway, it's a poor cowboy.
-
-
Let's lower the
saddle a little bit.
-
He cannot see the horse, OK?
-
So the saddle.
-
If I cross the saddle,
this is the saddle.
-
And these are his hands.
-
And he is holding his hat.
-
This is [INAUDIBLE].
-
And with one hand
is on the horse.
-
I don't know.
-
It's very [INAUDIBLE].
-
So what I'm trying to draw
looks something like this.
-
Right?
-
Eh.
-
Sorry.
-
More or less.
-
It's an abstract picture.
-
Very abstract picture.
-
So with this in mind, if I were
to look at the level curves,
-
I'm going to ask you,
what are the level curves?
-
Oh, my god, what are
the level curves?
-
-
You already have them in
your WeBWorK homework.
-
But for one point
extra credit, I
-
want you to draw
them on the floor.
-
Draw the level curves.
-
Remember what those were?
-
They were projections of
the curves on the surface
-
at the intersection
with z equals c planes.
-
You project them on the ground.
-
What do you think they are?
-
Think about it.
-
What are these?
-
If I take c, what
if c is positive?
-
-
What if c is 0?
-
What if c is less than 0?
-
What am I going to have?
-
Your imagination gives
you c equals 1, Magdalena.
-
Let's draw that.
-
OK.
-
Well, I'll try.
-
a and b would be 1, right, guys?
-
So a and b would be 1.
-
This is a square.
-
These would be the asymptotes.
-
So very, very
briefly, the hyperbola
-
would be this one-- x squared
minus y squared equals 1,
-
right?
-
If I have the last
case for c equals 1,
-
I'm going to have-- c
equals negative 1-- I'm
-
going to have the conjugate.
-
Are you guys with me?
-
So I'll have an a squared,
asymptotes, conjugate.
-
-
What if I have different level
c? c equals 1/2. c equals 2.
-
c equals pi. c
equals-- what are they?
-
I'm going to get
families of hyperbolas,
-
trenches that look like that.
-
Standard trenches and
conjugate trenches.
-
A multitude of them, an infinite
family of such hyperbolas,
-
an infinite family
of such hyperbolas.
-
I wanted to draw it.
-
What do I get when c is 0?
-
What are those?
-
STUDENT: Don't you
get, like, [INAUDIBLE]?
-
-
MAGDALENA TODA: They
get-- very good.
-
Why?
-
x squared minus y squared
equals 0 would lead
-
me to y equals plus/minus 1.
-
And who are those y
equals plus/minus 1?
-
Exactly.
-
But exactly the first
bisector, which is y equals x.
-
They are [? then the ?]
function.
-
And the other one, y equals
negative [? x. ?] So these
-
are the asymptotes.
-
So I'm going to get a-- you guys
have to do this better than me.
-
Sorry.
-
These are all
hyperbolic trenches.
-
They are all going to
infinity like that.
-
And I'm sorry that
I'm giving you
-
a little bit too many hints.
-
This is part of your
homework, your WeBWorK.
-
I shouldn't talk
too much about it.
-
-
Any questions so far?
-
Is this hard?
-
Yes, sir?
-
No.
-
STUDENT: So [? spherically, ?]
if you had z
-
equals y squared
minus x squared,
-
it's that same
picture, just flipped?
-
-
MAGDALENA TODA:
What would it be?
-
It would be the poor
saddle-- or cowboy--
-
STUDENT: Would be upside down.
-
MAGDALENA TODA:
--would be upside down.
-
Or projected in
something like a mirror.
-
I don't know how to say.
-
It would be exactly upside down.
-
So the reflection of that.
-
So you take all the points.
-
If you have-- I don't know.
-
It's hard to draw a reflection
in three dimensions.
-
But--
-
STUDENT: No, I understand.
-
MAGDALENA TODA:
Practically every curve
-
would be upside down with
respect to the floor.
-
OK.
-
All right.
-
I'm going to erase in one.
-
And you say, well, you've
taught us about these things,
-
like the domain and range.
-
But what about other notions,
like continuity and stuff?
-
-
Let me move on to 11.2.
-
Limits of functions of
the type z equals f of xy.
-
-
So what do you remember
about the limit
-
of a function of one variable?
-
Comparison.
-
-
What about the limit if you
take [? z's, ?] I don't know.
-
I should look stunned.
-
And I should be stunned.
-
Of a function of y equals
f of x of one variable.
-
-
When do we say that
f has a limit at a?
-
-
STUDENT: When the [INAUDIBLE]
approaches from the right
-
and the left to the same value.
-
MAGDALENA TODA: Actually, that
was the simpler definition.
-
Let's think a little bit deeper.
-
We say that f has a
limit L at x equals a.
-
That's kind of the idea,
left and right limits.
-
But not both of them
have to exist, you see.
-
Maybe only the limit
from the left or limit
-
from the right only exists.
-
-
If, for any choice of values
of x, closer and closer, closer
-
and closer to a, we get that
F gets closer and closer to L.
-
And this "any" I put in.
-
My god, I put it in
a red circle thing,
-
because one could get
subsequencies of a sequence.
-
And for that subsequence
thing, things
-
look like I would have a limit.
-
And then you say,
well, but in the end,
-
I don't have a limit, because
I can get another subsequence
-
of the sequence.
-
And for that one, I'm not
going to have a limit.
-
Can you give me an example
of some crazy function that
-
does not have a limit at 0?
-
Example of a crazy function.
-
No.
-
No, don't write "crazy."
-
Of a function f of x
that is not defined at 0
-
and does not have
limit at 0, although it
-
is defined for values
arbitrarily close to 0.
-
-
Moreover, I want that function
to be drawn without-- I
-
want the function to be
drawn without leaving
-
the paper when I draw.
-
-
[INAUDIBLE]
-
-
So something that would
be defined on the whole 0
-
infinity except for 0 that
I can draw continuously
-
except when I get to 0, I
get some really bad behavior.
-
I don't have a limit
for that function.
-
You are close to that.
-
Sine of 1/x.
-
STUDENT: I said y equals 1/x.
-
MAGDALENA TODA: y equals 1/x.
-
Very good.
-
Let's see.
-
STUDENT: Oh, yeah. [INAUDIBLE].
-
MAGDALENA TODA: Yeah, yeah.
-
Both are excellent examples.
-
So let's see.
-
This guy is a very
nice function.
-
How do we draw him, or her?
-
Well, it's a her, right?
-
It's a she.
-
It's a function.
-
No, no.
-
In English, it doesn't
make any sense,
-
but if I think French, Italian,
Spanish, Romanian-- now
-
I speak both Italian
and Romanian--
-
we say it's a she,
it's a feminine.
-
So as I approach with values
closer and closer and closer
-
to 0, what happens
to my poor function?
-
It blows up.
-
OK.
-
So I have limit of 1/x from
the right and from the left.
-
If I take it from the
left, I don't care.
-
Let's take it only
from the right.
-
OK?
-
-
It's close to 0.
-
That's going to blow up, right?
-
-
And I restrict it.
-
So let's say, if I want
the domain to be containing
-
[? both, ?] that's also fine.
-
So if you guys want, we
can draw the other one.
-
This goes to paradise.
-
The other one, I'm not
going to say where it goes.
-
But it's the same idea,
that as you approach 0
-
with closer and closer
and closer values,
-
it's going to blow up.
-
It's going to explode.
-
This is a beautiful function.
-
How beautiful [INAUDIBLE].
-
Beautiful with a
bad behavior near 0.
-
So I'm not going
to have a limit.
-
No limit.
-
Some people say, limit
exists and is infinity.
-
But does infinity exist?
-
Well, this is a
really philosophical,
-
religious notion, so I
don't want to get into it.
-
But in mathematics, we consider
that unless the limit is
-
finite, you cannot have a limit.
-
So if the limit is plus/minus
infinity, there is no limit.
-
Could the limit be different
or different subsequences?
-
This is what I
wanted to point out.
-
If you try this guy, you are
in real trouble on that guy.
-
Why?
-
You can have two.
-
If you have a graphing
calculator, which
-
I'm going to be opposed to you
being used in the classroom,
-
you would probably
see what happens.
-
Sine is defined on
all the real numbers.
-
But you cannot
have a value at 0,
-
because the 1/x is
not defined at 0.
-
Imagine you get closer and
closer to 0 from both sides.
-
I cannot draw very beautifully.
-
But as 1, this is plus
1 and this is minus 1.
-
I'm going to have some behavior.
-
And how many of you have seen
that on a computer screen
-
or calculator?
-
You've seen.
-
Yeah, you've seen.
-
By the way, did you
see the Lubbuck High?
-
Was it in high school you saw
it the first time in Calc 1
-
or pre-calc?
-
STUDENT: [INAUDIBLE]
Algebra 1 with Mr. West.
-
[INAUDIBLE]
-
MAGDALENA TODA: So I'll
try-- oh, guys, you
-
have to be patient with me.
-
I'm not leaving the poor board
with the tip of my pencil.
-
I'm not leaving him.
-
I have continuity.
-
As I got closer to this, I still
have the [INAUDIBLE] property.
-
Anyway, it's OK.
-
I'm not leaving this.
-
I am taking all the values
possible between minus 1 and 1.
-
So on intervals that
are smaller, smaller,
-
I'm really taking all the
values between minus 1 and 1,
-
and really rapidly--
[INAUDIBLE].
-
When I'm getting closer to 0,
I'm not going to have a limit.
-
But as somebody
may say, but wait.
-
When I have a sequence
of values that
-
is getting closer
and closer to 0,
-
is that no guarantee that
I'm going to have a limit?
-
Nope.
-
It depends.
-
If you say "any," it has to
be for any choice of points,
-
any choice of points
that you go closer to 0.
-
Not for one sequence
of points that
-
is getting closer
and closer to 0.
-
For example, if your
choice of points is this,
-
choice of points.
-
-
Getting closer to 0.
-
[INAUDIBLE] xn
equals 1 over 2 pi n.
-
Isn't this going to 0?
-
Yeah.
-
It then goes to infinity.
-
This sequence goes to 0.
-
What is it?
-
1 over 2 pi?
-
1 over 4 pi?
-
1 over 8 pi?
-
1 over 16 pi?
-
1 over 32 pi?
-
1 over 64 pi?
-
This is what my
son is doing to me.
-
And I say, please stop.
-
OK?
-
He's 10 years old.
-
He's so funny.
-
Now, another choice of points.
-
-
Ah.
-
Somebody-- all of you are
smart enough to do this.
-
What do you think
I'm going to pick?
-
1 over what?
-
And when [? other ?]
something that goes to 0
-
then goes to infinity.
-
And I know that your
professor showed you that.
-
pi over 2 plus 2 pi n.
-
Doesn't this go to 0?
-
Yes.
-
As n gets bigger and
bigger, this is going to 0.
-
However, there is no limit.
-
Why?
-
Well, for the first sequence,
as xn goes to 0, f of xn
-
goes to-- what is
sine of-- OK, I
-
am too lazy to write this down.
-
Sine of 1 over 1 over--
of 1 over 1 over 2 pi?
-
-
STUDENT: It's the
sine over 2 pi.
-
MAGDALENA TODA: This
is sine of 2 pi n.
-
And how much is that?
-
STUDENT: 0.
-
MAGDALENA TODA: 0.
-
So this is a 0.
-
And this is a--
this converges to 0.
-
So I say, oh, so maybe I have
a limit, and that'll be 0.
-
Wrong.
-
That would be the rapid,
stupid conclusion.
-
If somebody jumps [? up, ?]
I picked some points,
-
I formed the sequence that
gets closer and closer to 0.
-
I'm sure that the limit exists.
-
I've got a 0.
-
Well, did you think of
any possible choice?
-
That's the problem.
-
You have to have
any possible choice.
-
F of yn sine of 1 over
pi over 2 plus 1 over 1
-
over-- Magdalena-- pi
over 2 plus 2 pi n.
-
So we saw that this was 0.
-
What happens to sine of
1 over 1 over sine of pi
-
over 2 plus 2 pi n?
-
And where does this go?
-
It then goes to infinity.
-
-
This sequence goes to 0.
-
What is f of the
sequence going to?
-
To another limit.
-
So there is no limit.
-
What's the limit of
this subsequence?
-
It's a constant one, right?
-
Because look, what does it
mean pi over 2 plus 2 pi n?
-
Where am I on the unit
trigonometric circle?
-
[INTERPOSING VOICES]
-
Always here, right?
-
Always on the sort of
like the north pole.
-
So what is the sine
of this north pole?
-
STUDENT: 1.
-
MAGDALENA TODA: Always 1.
-
So I get the limit 1.
-
So I'm done because there
are two different limits.
-
So pay attention to
this type of problem.
-
Somebody can get you in trouble
with this kind of thing.
-
On the other hand,
I'm asking you,
-
what if I want to make this
a function of two variables?
-
-
So I'll say, one
point extra credit.
-
I'm giving you too
much extra credit.
-
Maybe I give you
too much-- it's OK.
-
One point extra credit--
put them together.
-
-
Does f-- do you
like to do the f?
-
I used big F, and then I
changed it to little f.
-
This time I have a function
of two variables-- little
-
f with xy-- to be sine of 1
over x squared plus y squared.
-
Does this function have a
limit at the point 0, 0?
-
-
So when I approach 0,
0, do I have a limit?
-
OK.
-
And you say, well, it depends
how I approach that 0, 0.
-
That's exactly the thing.
-
Yes, sir.
-
Oh, you didn't want to ask me.
-
-
And does f of xy
equals-- let me give you
-
another one, a
really sexy one. x
-
squared plus y squared
times sine of 1
-
over x squared plus y squared.
-
Have a limit at 0, 0?
-
-
I don't know.
-
Continuous it cannot be,
because it's not defined there.
-
Right?
-
For a function to be
continuous at a point,
-
the function has to
satisfy three conditions.
-
The function has to be
defined there at that point.
-
The function has to
have a limit there
-
at that point of the domain.
-
And the limit and the function
value have to coincide.
-
Three conditions.
-
We will talk about
continuity later.
-
Hint.
-
Magdalena, too many hints.
-
This should remind
you of somebody
-
from the first
variable calculus.
-
It's a more challenging problem.
-
That's why I gave
it to extra credit.
-
If I had x sine of 1/x,
what would that look like?
-
STUDENT: x times--
-
MAGDALENA TODA: x
times sine of 1/x.
-
When I approach 0
with-- so if I have-- I
-
don't ask for an answer now.
-
You go home, you think about it.
-
You take the calculator.
-
But keep in mind that your
calculator can fool you.
-
Sometimes it can show an
image that misguides you.
-
So you have to think
how to do that.
-
How about x times
sine of 1/x when--
-
does it have a limit
when x goes to 0?
-
Is there such a limit?
-
Does it exist?
-
-
So if such a limit
would exist, maybe we
-
can extend by continuity the
function x times sine over x.
-
What does it mean?
-
Like, extend it, prolong it.
-
And say, it's this 4x equals
0 and this if x is not 0.
-
So this is obviously x is
different from 0, right?
-
Can we extend it by continuity?
-
Think about the drawing.
-
Think about the arguments.
-
And I think it's time for me
to keep the promise I made
-
to [? Aaron, ?]
because I see no way.
-
Oh, my god, [? Aaron, ?]
I see no way out.
-
-
The epsilon delta
definition of limit.
-
[? Right? ?] OK.
-
So what does it mean for a
real mathematician or somebody
-
with a strong mathematical
foundation and education
-
that they know the
true definition
-
of a limit of a function
of, let's say, one variable?
-
The epsilon delta, the one your
dad told you about. [INAUDIBLE]
-
try to fool you when avoid it
in undergraduate education.
-
People try to avoid
the epsilon delta,
-
because they think the students
will never, never understand
-
it, because it's
such an abstract one.
-
I think I wasn't ready.
-
I wasn't smart enough.
-
I think I was 16 when I was
getting ready for some math
-
competitions.
-
And one professor taught me
the epsilon delta and said,
-
do you understand it?
-
My 16-year-old mind said, no.
-
But guess what?
-
Some other people
smarter than me,
-
they told me, when
you first see it,
-
you don't understand
it in any case.
-
So it takes a little bit
more time to sink in.
-
So the same idea.
-
As I'm getting closer and
closer and closer and closer
-
to an x0 with my x values
from anywhere around-- left,
-
right-- I have to pick an
arbitrary choice of points
-
going towards x0, I have to
be sure that at the same time,
-
the corresponding sequence
of values is going to L,
-
I can express that
in epsilon delta.
-
-
So we say that.
-
-
f of x has limit L
at x equals x0 if.
-
-
For every epsilon positive, any
choice of an epsilon positive,
-
there is a delta.
-
There exists-- oh, OK, guys.
-
You don't know the symbols.
-
I'll write it in English.
-
For every epsilon
positive, no matter
-
how small-- put
parentheses, because you
-
are just [? tired-- ?]
no matter how small,
-
there exists a delta number
that depends on epsilon.
-
-
So that whenever x minus
x0 is less than delta,
-
this would imply
that f of x minus L,
-
that limit I taught you
about in absolute value,
-
is less than epsilon.
-
-
What does this mean?
-
I'm going to try
and draw something
-
that happens on a line.
-
So this is x0.
-
And these are my values of x.
-
They can come from anywhere.
-
-
And this is f of x.
-
And this is L. So it
says, no matter-- this
-
says-- this is an
abstract way of saying,
-
no matter how close, you see,
for every epsilon positive,
-
no matter how close
you get to the L.
-
I decide to be in this
interval, very tiny epsilon.
-
L minus epsilon.
-
L plus epsilon L. You give
me your favorite epsilon.
-
You say, Magdalena, pick
something really small.
-
Big epsilon to be 0.00001.
-
How about that?
-
Well, if I really
have a limit there,
-
an L at x0, that means that
no matter how much you shrink
-
this interval for me, you
can be mean and shrink it
-
as much as you want.
-
I will still find a
small interval around x0.
-
-
[? But ?] I will
still find the smaller
-
interval around x0, which is--
this would be x0 minus delta.
-
This would be x0 plus delta.
-
So that the image of this
purple interval fits inside.
-
You say, what?
-
So that the image of this
purple interval fits inside.
-
So f of x minus L, the distance
is still that, less than xy.
-
Yes, sir?
-
STUDENT: Where'd you
get epsilon [INAUDIBLE]?
-
MAGDALENA TODA: So
epsilon has to be
-
chose no matter how small.
-
STUDENT: [INAUDIBLE].
-
MAGDALENA TODA: Huh?
-
Real number.
-
So I'm saying, you should not
set the epsilon to be 0.0001.
-
That would be a mistake.
-
You have to think of that number
as being as small as you want,
-
infinitesimally small, smaller
than any particle in physics
-
that you are aware about.
-
And this is what I had the
problem understanding--
-
that notion of-- not
the notion of, hey, not
-
matter how close
I am, I can still
-
get something even smaller
around x0 that fits in this.
-
That's not what I
had the problem with.
-
The notion is to perceive
an infinitesimal.
-
Our mind is too limited
to understand infinity.
-
It's like trying
to understand God.
-
And the same limitation comes
with microscopic problems.
-
Yeah, we can see some
things on the microscope,
-
and we understand.
-
Ah, I understand I
have this bacteria.
-
This is staph.
-
Oh, my god.
-
But then there are molecules,
atoms, subatomic particles
-
that we don't understand,
because our mind is really
-
[? small. ?] Imagine
something smaller
-
than the subatomic particles.
-
That's the abstract notion
of infinitesimally small.
-
So I'm saying, if I really
have a limit L there,
-
that means no matter how small
I have this ball around it,
-
I can still find a
smaller ball that
-
fits-- whose image fits inside.
-
All right?
-
The same kind of definition--
I will try to generalize this.
-
Can you guys help me
generalize this limit notion
-
to the notion of function
of two variables?
-
-
So we say, that f of xy
has the limit L at x0y0.
-
-
What was x0y0 when I
talked about-- what
-
example did I give you guys?
-
Sine of 1 over x squared
plus y squared, right?
-
Something like that.
-
I don't know.
-
I said, think of 0, 0.
-
That was the given point.
-
It has to be a fixed couple.
-
So you think of the origin, 0,
0, as being as a fixed couple.
-
Or you think of the point 1,
0 as being as a fixed couple
-
in that plane you look at.
-
That is the fixed couple.
-
If-- now somebody
has to help me.
-
For every epsilon positive,
no matter how small,
-
that's where I have a problem
imagining infinitesimally
-
small.
-
There exists-- I no
longer have this problem.
-
But I had it enough
when I was in my 20s.
-
I don't want to go back to
my 20s and have-- I mean,
-
I would love to.
-
[LAUGHTER]
-
To go having vacations
with no worries and so on.
-
But I wouldn't like
to go back to my 20s
-
and have to relearn
all the mathematics.
-
Now way.
-
That was too much of a struggle.
-
There exists a delta positive
that depends on epsilon.
-
What does it mean,
depends on epsilon?
-
Because guys, imagine
you make this epsilon
-
smaller and smaller.
-
You have to make delta
smaller and smaller,
-
so that you can fit that
little ball in the big ball.
-
OK?
-
That depends on epsilon,
so that whenever-- now,
-
that is a big problem.
-
How do I say, distance between
the point xy and the point
-
x0y0?
-
Oh, my god.
-
This is distance between xy
and x0y0 is less than delta.
-
This would imply
that-- well, this
-
is a function with values in
R. This is in R. Real number.
-
So I don't have a problem.
-
I can use absolute value here.
-
Absolute value of f of
the couple xy minus L
-
is less than epsilon.
-
The thing is, can you
visualize that little ball,
-
that little disk?
-
What do I mean?
-
Being close, xy is me, right?
-
But I'm moving.
-
I'm the moving point.
-
I'm dancing around.
-
And [? Nateesh ?] is x0y0.
-
How do I say that I have
to be close enough to him?
-
I cannot touch him.
-
That's against the rules.
-
That's considered
[INAUDIBLE] harassment.
-
But I can come as
close as I want.
-
So I say, the
distance between me--
-
I'm xy-- and
[? Nateesh, ?] who is
-
fixed x0y0, has to be smaller
than that small delta.
-
How do I represent that
in plane mathematics?
-
STUDENT: Doesn't [INAUDIBLE]?
-
-
MAGDALENA TODA: Exactly.
-
So that delta has to
be small enough so
-
that the image of f at me minus
the limit is less than epsilon.
-
Now you understand why all
the other teachers avoid
-
talking about this
[? one. ?] So I
-
want to get small enough-- not
too close-- but close enough
-
to him, so that my value--
I'm f of xy-- minus the limit,
-
the limit-- I have
a preset limit.
-
All around [? Nateesh, ?] I
can have different values,
-
no matter where I go.
-
My value at all these points
around [? Nateesh ?] have
-
to be close enough
to L. So I say,
-
well, you have to get
close enough to L.
-
Somebody presents me an epsilon.
-
Then I have to reduce my
distance to [? Nateesh ?]
-
depending to that epsilon.
-
Because otherwise,
the image doesn't fit.
-
It's a little bit tricky.
-
STUDENT: So is this like
the squeeze theorem kind of?
-
MAGDALENA TODA: It is
the squeeze theorem.
-
STUDENT: Oh, all right.
-
MAGDALENA TODA: OK?
-
So the squeezing-- I ball into
another [? ball ?] [? limit. ?]
-
This is why-- it's not
a ball, but it's a--
-
STUDENT: A circle.
-
MAGDALENA TODA: Disk.
-
A circle, right?
-
So how do we express
that in Calc 3 in plain?
-
This is the
[? ingredient, ?] distance d.
-
So Seth, can you tell me what is
the distance between these two
-
points?
-
Square root of--
-
STUDENT: [INAUDIBLE].
-
MAGDALENA TODA: x minus
x0 squared plus y minus y0
-
squared.
-
Now shut up. [? And I ?]
am talking to myself.
-
STUDENT: Must be
less than delta.
-
[LAUGHTER]
-
MAGDALENA TODA: Less than delta.
-
So instead of
writing this, I need
-
to write that I can
do that in my mind.
-
-
OK?
-
All right.
-
This is hard.
-
We need to sleep on that.
-
I have one or two more problems
that are less hard-- nah,
-
they are still hard, but
they are more intuitive,
-
that I would like to
ask you about the limit.
-
I'm going to give
you a function.
-
And we would have to visualize
as I get closer to a point
-
where I am actually going.
-
So I have this nasty
function, f of xy
-
equals xy over z
squared plus y squared.
-
-
And I'm saying, [INAUDIBLE]
the point is the origin.
-
I choose the origin.
-
Question.
-
Do I have a limit that's--
do I have a limit?
-
Not [? really ?] for me.
-
Does f have a limit
at the origin?
-
-
You would have to imagine
that you'd draw this function.
-
And except you cannot draw, and
you really don't care to draw
-
it.
-
You only have to imagine that
you have some abstract graph--
-
z equals f of xy.
-
You don't care
what it looks like.
-
But then you take
points on the floor,
-
just like I did the exercise
with [? Nateesh ?] before.
-
And you get closer and
closer to the origin.
-
But no attention-- no
matter what path I take,
-
I have to get the same limit.
-
What?
-
No matter what path I take
towards [? Nateesh-- ?]
-
don't write that down-- towards
[? z0y0, ?] I have to get
-
the same limit.
-
-
Do I?
-
Let's imagine with the
eyes of your imaginations.
-
And [? Nateesh ?]
is the point 0, 0.
-
And you are aspiring to get
closer and closer to him
-
without touching him.
-
Because otherwise,
he's going to sue you.
-
So what do we have here?
-
We have different paths?
-
How can I get closer?
-
Either on this path
or maybe on this path.
-
Or maybe on this path.
-
Or maybe, if I had something
to drink last night-- which
-
I did not, because
after the age of 35,
-
I stopped drinking completely.
-
-
That's when I decided
I want to be a mom,
-
and I didn't want to
make a bad example.
-
So no matter what path you
take, you can make it wiggly,
-
you can make it
any way you want.
-
We are still approaching 0, 0.
-
You still have to
get the same limit.
-
Oh, that's tricky, because
it's also the same in life.
-
Depending on the path
you take in life,
-
you have different
results, different limits.
-
Now, what if I take the path
number one, number two, number
-
three possibility.
-
And number [? blooie ?]
is the drunken variant.
-
That is hard to
implement in an exercise.
-
Imagine that I have
limit along the path one.
-
Path one.
-
xy goes to 0, 0 of xy over
x squared plus y squared.
-
Do you guys see what's
going to happen?
-
So I'm along the--
OK, here it is.
-
This line, right, this is
the x-axis, y-axis, z-axis.
-
What's special for the x-axis?
-
Who is 0?
-
STUDENT: x.
-
STUDENT: yz.
-
MAGDALENA TODA: y is 0.
-
So y is 0.
-
So y is 0.
-
Don't laugh at me.
-
I'm going to write like
that because it's easier.
-
And it's going to be
something like limit
-
when x approaches 0
of x over x squared.
-
STUDENT: It's 1/x.
-
MAGDALENA TODA: Times 0 up.
-
Oh, my god.
-
Is that-- how much is that?
-
STUDENT: 0.
-
STUDENT: 0.
-
MAGDALENA TODA: 0!
-
I'm happy.
-
I say, maybe I have the limit.
-
I have the limit 0.
-
No, never rush in life.
-
Check.
-
Experiment any other paths.
-
And it's actually very easy
to see where I can go wrong.
-
If I take the path number two,
I will get the same result.
-
You don't need a
lot of imagination
-
to realize, hey, whether
she does it for x
-
or does it for y, if she
goes along the 2, what
-
the heck is going to happen?
-
y is going to shrink.
-
x will always be 0.
-
Because this means
a point's like what?
-
0,1.
-
0, 1/2.
-
0, 1/n, and so on.
-
But plug them all in here,
I get 0, 1/n times 0.
-
It's still 0.
-
So I still get 0.
-
Path two.
-
When I approach my--
xt goes to 0, 0.
-
The poor [? Nateesh ?]
is waiting for an answer.
-
I still get 0.
-
Let's take not the
drunken path, because I
-
don't know [? it unless ?]
the sine function.
-
That is really crazy.
-
I'll take this one.
-
What is this one,
in your opinion?
-
Is that going to help me?
-
I don't know, but I
need some intuition.
-
Mathematicians need intuition
and a lot of patience.
-
So what is your intuition?
-
The one in the middle, I'm going
to start walking on that, OK,
-
until you tell me what it is.
-
STUDENT: y [INAUDIBLE].
-
MAGDALENA TODA: y equals
x is the first bisector
-
or the first quadrant.
-
And I'm very happy
I can go both ways.
-
y equals x.
-
x [INAUDIBLE].
-
So limit when x equals y,
but the pair xy goes to 0,0.
-
I'm silly.
-
I can say that,
well, Magdalena, this
-
is the pair xx,
because x equals what?
-
Let me plug them in.
-
So it's like two people.
-
x and y are married.
-
They are a couple, a pair.
-
They look identical.
-
Sometimes it happens.
-
Like twins, they
start looking alike,
-
dressing alike, and so on.
-
The x and the y have to
receive the same letter.
-
And you have to tell me what
in the world the limit will be.
-
-
STUDENT: 1/2.
-
MAGDALENA TODA: 1/2.
-
Oh, my god.
-
So now I'm deflated.
-
So now I realize that
taking two different paths,
-
I show that I have-- on
this path, I have 1/2.
-
On this path, I have 0.
-
I don't match.
-
I don't have an overall limit.
-
So the answer is,
no overall limit.
-
Oh, my god.
-
So what you need to
do, guys, is read
-
section 11.1 and section 11.2.
-
And I will ask you next
time-- and you can lie,
-
you can do whatever.
-
Did the book explain
better than me,
-
or I explain better
than the book?
-
This type of example when
the limit does not exist.
-
We are going to
see more examples.
-
You are going to see examples
where the limit does exist.
-
Now, one last thing.
-
When you have to compute limits
of compositions of functions
-
whose limit exist--
for example, you
-
know that limit is
xy goes to x0y0 of f
-
of xy [INAUDIBLE] limit
of xy go to x0y0 of gxy
-
is L-- L-- L-- M-- M.
-
How are you going to compute the
limit of alpha f plus beta g?
-
This is in the book.
-
But you don't need the
book to understand that.
-
You will already
give me the answer,
-
because this is the equivalent
thing to the function of one
-
variable thing in Calc 1.
-
So if you would only
have f of x or g of x,
-
it would be piece of cake.
-
What would you say?
-
STUDENT: [INAUDIBLE].
-
MAGDALENA TODA: Right.
-
Alpha times L plus beta
times M. Can you also
-
multiply functions.
-
Yes, you can.
-
Limit of fg as xy goes
to x0 or y0-- will be LM.
-
How about-- now I'm going to
jump to conclusion, hoping
-
that you are going to catch me.
-
You are going to catch
me, and shout at me,
-
and say, ooh, pay
attention, Magdalena,
-
you can make a mistake there.
-
I say it's L/M when I do
the division rule, right?
-
Where should I pay attention?
-
STUDENT: M [INAUDIBLE].
-
MAGDALENA TODA: Pay attention.
-
Sometimes you can
have the-- right?
-
And this also has
to exist as well.
-
STUDENT: [INAUDIBLE].
-
MAGDALENA TODA: So one
last-- how many minutes
-
have I spent with you?
-
I've spent with you a long
number of hours of my life.
-
No, I'm just kidding.
-
So you have one hour and
15, a little bit more.
-
Do I have a little bit more?
-
Yes.
-
I have 15 minutes.
-
I have--
-
STUDENT: So we get out at--
-
[INTERPOSING VOICES]
-
MAGDALENA TODA: 50.
-
Five more minutes.
-
OK.
-
So I want to ask you what
you remember about some
-
of your friends, the trig
functions involved in limits.
-
-
Why did we study
limits at the point
-
where the function's
not defined?
-
Well, to heck with it.
-
We don't care.
-
The function is
not defined at 0.
-
But the limit is.
-
And nobody showed you how
to do the epsilon delta
-
to show anything like that.
-
-
OK.
-
Can you do that
with epsilon delta?
-
-
Actually, you can do
everything with epsilon delta.
-
But I'm not going to give
you any extra credit.
-
So I trust you that
you remember that.
-
1!
-
How about-- let me-- OK.
-
I am so proud of you.
-
Let me challenge you more.
-
Let me challenge you more.
-
Tangent of ax over bx.
-
x go to 0.
-
I asked this to a girl
from Lubbock High.
-
She was in high school.
-
She knew the answer.
-
STUDENT: Oh, I can't disappoint
everybody in getting this.
-
STUDENT: Is it 1/a?
-
Oh, I can't remember.
-
MAGDALENA TODA: Tell me
what to do to be smart.
-
Right?
-
I have to be doing
something smart.
-
She-- can you give me hint?
-
I'm your student
and you say, well--
-
STUDENT: ba--
-
STUDENT: It's 0.
-
STUDENT: It's [INAUDIBLE].
-
MAGDALENA TODA: Um, it's a what?
-
STUDENT: b/a?
-
MAGDALENA TODA: I'm
not [INAUDIBLE].
-
I don't think so.
-
So what should I do?
-
I should say, instead of
bx-- that drives me nuts.
-
This goes on my nerves-- bx.
-
Like, maybe I go on your
nerves. bx is ax, right?
-
If it were ax, I would
be more constructive,
-
and I knew what to do.
-
I say replace bx with
ax, compensate for it,
-
and divide by bx.
-
And I was trying to
explain that to my son,
-
that if you have a fraction
a/b, and then you write a/n
-
times n/b, it's the same thing.
-
Gosh, I had the
problem with him.
-
And then I realized that he
didn't do simplifications
-
in school.
-
So it took a little more
hours to explain these things.
-
This is fourth grade.
-
I think I remember doing
that in fourth grade.
-
Third grade, actually.
-
So these two guys disappear.
-
I haven't changed
my problem at all.
-
But I've changed the status,
the shape of my problem
-
to something I can mold,
because this goes to somebody,
-
and this goes to somebody else.
-
Who is this fellow?
-
It's a limit that's
a constant-- a/b.
-
Who is this fellow?
-
STUDENT: 1.
-
MAGDALENA TODA: 1.
-
Because tangent of x/x as x
goes to 0 goes to 1 exactly
-
like that.
-
So limit of sine x over cosine
x, that's tangent, right?
-
Over x.
-
You do it exactly the same.
-
It's limit of sine x/x
times 1 over cosine x.
-
That's how we did
it in high school.
-
This goes to 1.
-
This goes to 1.
-
So it's 1.
-
So thank you, this is 1.
-
I know I took a little more time
to explain than I wanted to.
-
But now you are grown up.
-
In two minutes, you are
going to be finishing
-
this section, more or less.
-
What if I put a function
of two variables,
-
and I ask you what
the limit will be,
-
if it's the same
type of function.
-
So you say, oh, Magdalena,
what you doing to us?
-
OK, we'll see it's fun.
-
This one's fun.
-
It's not like the one before.
-
This one is pretty beautiful.
-
It's nice to you.
-
It exists.
-
xy goes to 0, 0.
-
So you have to imagine
some preferable function
-
in abstract thinking.
-
And you want it in
a little disk here.
-
And xy, these are all points
xy close enough to 0, 0,
-
in the neighborhood of 0, 0.
-
OK.
-
What's going to happen as
you get closer and closer
-
and closer and closer with
tinier and tinier and tinier
-
disks around 0, 0?
-
You're going to shrink so much.
-
What do you think
this will going to be,
-
and how do I prove it?
-
STUDENT: [INAUDIBLE].
-
MAGDALENA TODA: Who said it?
-
You, sir? [INAUDIBLE]
going to go to 1.
-
And he's right.
-
He has the intuition.
-
A mathematician will
tell you, prove it.
-
STUDENT: Um, well,
let's see here.
-
MAGDALENA TODA: Can you prove?
-
STUDENT: You could use
the right triangle proof,
-
but that would probably
take way more [INAUDIBLE].
-
MAGDALENA TODA: x and
y are independent.
-
That's the problem.
-
They are married, but they
are still independent.
-
It's a couple.
-
However, we can use
polar coordinates.
-
Why is polar coordinates?
-
Well, in general, if we
are in xy, it's a pair.
-
This is r, right?
-
So rx is r cosine theta.
-
y is r sine theta.
-
And I can get closer and
closer to the original.
-
I don't care.
-
What happens about x
squared plus y squared,
-
this is r squared.
-
And r is a real number.
-
And as you walk closer
and closer to the original
-
without touching it,
that r goes to 0.
-
It shrinks to 0.
-
So that r squared goes
to 0 but never touches 0.
-
So this becomes limit as r goes
to 0, the radius of that disk
-
goes to 0.
-
Sine of r squared
over r squared.
-
But r squared could be replaced
by the real function, t,
-
by the real parameter,
lambda, by whatever you want.
-
So then it's 1.
-
And then Alexander was right.
-
He based it on, like,
observation, intuition,
-
everything you want.
-
It was not a proof.
-
On a multiple-choice exam,
he would be a lucky guy.
-
I don't want you to prove it.
-
But if I want you to
prove it, you have to say,
-
Magdalena, I know
polar coordinates,
-
and so I can do it.
-
And one last question for today.
-
Guys, I'm asking you,
limit xy goes to 0, 0.
-
You will see some of these in
your WeBWorK for Chapter 11
-
that's waiting for
you, homework 3.
-
Tangent of 2 x squared
plus y squared over 3
-
x squared plus y squared.
-
What is that?
-
2/3.
-
STUDENT: 2/3.
-
MAGDALENA TODA: Am
I asking you why?
-
No, enough.
-
OK.
-
[INAUDIBLE] I gave
you everything
-
you need to show that.
-
x squared plus y squared,
again, is Mr. r squared.
-
It's OK.
-
I taught you that.
a/b. a is 2, b is 3.
-
Is it hard?
-
It is not easy, for sure.
-
Calc 3 is really difficult
compared to other topics
-
you are probably taking.
-
But I hope that I
can convince you
-
that math, although
difficult, [INAUDIBLE] Calc 3,
-
is also fun.
-
OK?
-
All right.
-
So I need attendance and
I need the extra credit.
-
STUDENT: Yeah, [INAUDIBLE].
-
-
MAGDALENA TODA: Before
you go, you need to sign.
-