-
So we're told that
AE is equal to 12.
-
That's this side
right over here.
-
And EC is equal to 18.
-
And then they've drawn a bunch
of the medians here for us.
-
So we know that they are medians
because, when they intersect
-
the opposite side, they're
telling us that this length is
-
equal to this length, or
that ED is equal to DC,
-
CB is equal to BA,
AF is equal to FE,
-
or that F, B, and
D are the midpoints
-
and that G, then, would
be the centroid where
-
the medians intersect.
-
And so the first thing they ask
us is, what is the area of BGC?
-
So BGC right here.
-
That is this triangle
right over there.
-
And to figure out
that area, we just
-
have to remind ourselves that
the three medians of a triangle
-
divide a triangle into six
triangles that have equal area.
-
So if we know the area of the
entire triangle-- and I think
-
we can figure this out.
-
This is a right triangle.
-
They're telling us that.
-
AE-- this entire distance right
over here-- is going to be 12.
-
So this is going to be 12.
-
Let me make sure I
have enough space.
-
This entire distance
right over here is 18.
-
They tell us that.
-
So the area of
AEC is going to be
-
equal to 1/2 times
the base-- which
-
is 18-- times the height--
which is 12-- which
-
is equal to 9 times
12, which is 108.
-
That's the area of this entire
right triangle, triangle AEC.
-
If we want the area of BGC or
any of these smaller of the six
-
triangles-- if we ignore
this little altitude
-
right over here, the ones that
are bounded by the medians--
-
then we just have
to divide this by 6.
-
Because they all
have equal area.
-
We've proven that
in a previous video.
-
So the area of BGC is
equal to the area of AEC,
-
the entire triangle, divided by
6, which is 108 divided by 6.
-
Which is what?
-
It's 60-- let's see.
-
You get 10 and then 48.
-
Looks like it would be 18.
-
It would be 18.
-
And that's right
because it would
-
be-- 108 is the same
thing as 18 times 6.
-
So we did our first part.
-
The area of that right
over there is 18.
-
And if we wanted,
we could say, hey,
-
the area of any of
these triangles--
-
the ones that are bounded
by the medians-- this
-
is going to be 18.
-
This is going to be 18.
-
This entire FGE triangle
is going to be 18,
-
but we did this first
part right over there.
-
Now they ask us, what
is the length of AG?
-
So AG is the distance.
-
It's the longer part of
this median right over here.
-
And to figure out
what AG is, we just
-
have to remind ourselves
that the centroid is always
-
2/3 along the way
of the medians,
-
or it divides the
median into two segments
-
that have a ratio of 2 to 1.
-
So if we know the entire
length of this median,
-
we could just take 2/3 of that.
-
And that'll give us
the length of AG.
-
And lucky for us, this
is a right triangle.
-
And we know that F and
D are the midpoints.
-
So for example, we
know this AE is 12.
-
That was given.
-
We know that ED is
half of this 18.
-
So ED right over here--
I'll do this in a new color.
-
ED is going to be 9.
-
So then we could just use
the Pythagorean theorem
-
to figure out what AD is.
-
AD is the hypotenuse
of this right triangle.
-
So we're looking at
triangle AED right now.
-
Let me write this down.
-
We know that 12
squared plus 9 squared
-
is going to be
equal to AD squared.
-
12 squared is 144.
-
144 plus 81.
-
And so this is going to
be equal to AD squared.
-
So this is what?
-
This is 225.
-
So we have 225 is
equal to AD squared.
-
And 225, you may or may not
recognize, is 15 squared.
-
So AD is equal to 15.
-
You want to take the principal
root, the positive root,
-
because we're talking about
distances or lengths of sides.
-
We don't care about
the negatives.
-
So AD is equal to 15.
-
So this whole thing
right over here
-
is going to be equal to 15.
-
And AG is going to be 2/3 of AD.
-
We proved that in
a previous video,
-
that the centroid is
2/3 along the way of any
-
of these medians.
-
And we could do it for
any of the medians.
-
So it's equal to 2/3 times
15, which is equal to 10.
-
So AG right over
here is equal to 10.
-
So we did the second part.
-
Now, this third part,
what is the area of FGH?
-
So let me color it in-- FGH.
-
So if we knew this length-- if
we knew HG and if we knew FH--
-
we could easily figure
out what that area is.
-
And there's actually
multiple ways
-
of figuring out either
one of those things.
-
So one way that we can think
about finding what HG is
-
is to remind
ourselves that HG is
-
the altitude of either
triangle FGE or triangle AFG.
-
And both of them
have a base of 6.
-
So this is 6, and
this is 6 over here.
-
And then they have a
height equal to GH.
-
And we know what the area is.
-
We know that the area
is already equal to 18.
-
So let's take this
triangle up here.
-
So we're talking about
the area of triangle AFG.
-
So we know it's 1/2
times its base, which
-
is 6, times its height,
which is GH-- times GH.
-
That's just 1/2
base times height
-
is equal to the area
of this triangle, which
-
is going to be equal to 18.
-
And so then, we just have
to tell ourselves, well,
-
this is-- 3 times
GH is equal to 18.
-
If we divide both sides of
this by 3, GH is equal to 6.
-
So that is one way to do it.
-
GH is equal to 6.
-
You could have also made
the similarity argument.
-
Then you could have said,
look, this triangle up here
-
is similar to this larger
triangle over here.
-
This hypotenuse is 2/3 of the
length of this entire thing.
-
So this is going to
be 2/3 of this 9.
-
So that's another way that
you could have gotten 6 there.
-
But either way, we
got this length.
-
Now we just have to
figure out what FH is.
-
And we could figure out what
FH is if we know what AH is.
-
Because we know A to F is 6.
-
So FH is going to
be AH minus AF.
-
So let's figure out what AH is.
-
Well, once again, we can
make a similarity argument.
-
And if we want to
do it formally,
-
we see that both this larger
right triangle and the smaller
-
right triangle, both have
a 90-degree angle there.
-
They both have this
angle in common.
-
So they have two
angles in common.
-
They are definitely
similar triangles.
-
And so we know the ratio of
AH-- let me do it in orange.
-
We know that the ratio
of AH to AE-- which
-
is 12-- is equal to
the ratio of AG-- which
-
is 10-- to the ratio of AD--
which we already figured out
-
was 15.
-
So one way to think about it
is AH is going to be 2/3 of 12.
-
Or we can just work
through the math just using
-
the similar triangles.
-
So this right-hand side
over here is just 2/3.
-
And so AH-- multiplying
both sides by 12--
-
is equal to 2/3 times
12, which is just 8.
-
So AH here is 8.
-
AH is 8.
-
AF is 6.
-
So FH right over here
is going to be 2.
-
And so now we have
enough information
-
to figure out the area of FHG.
-
So let me write it over here.
-
It's going to be
1/2 times the base.
-
I'll just use FH
as the base here,
-
although I could
do it either way.
-
Well, I'll use FH as the base.
-
1/2 times 2 times the height--
times 6-- which is equal to 6.
-
And we are done.
-
And you could keep going.
-
You could figure out the
length of pretty much
-
all of these segments here
using some of these techniques
-
or any of these areas.
-
Well, we've actually
figured out most of them.