Identifying a u in u substitution

- [Tutor] What we're
going to do in this video

is give ourselves some practice

in the first step of u substitution,

which is often the most difficult

for those who are first learning it

and that's recognizing when
u substitution is appropriate

and then defining an appropriate u.

So let's just start with an example here,

so let's say we wanna take
the indefinite integral

of two x plus one times

the square root of x squared plus x, dx,

does u substitution apply
here and if it does,

how would you define the u?

Pause the video and try
to think about that.

Well, we just have to remind ourselves,

that u substitution is really
trying to undo the chain rule,

if we remind ourselves what
the chain rule tells us,

it says look, if we have
a composite function,

let's say f of g of x,

f of g of x and we take
the derivative of that

with respect to x,

that that is going to be equal

to the derivative of the outside function

with respect to the inside function,

so f prime

of g of x

times the derivative
of the inside function,

times the derivative
of the inside function

and so u substitution is all about

well, do we see a pattern
like that inside the integral?

Do we see a potential
inside function, a g of x,

where I see its derivative
being multiplied?

Well, we see that over here,

if I look at x squared plus x,

if I make that the u, what's
the derivative of that?

Well, the derivative of x
squared plus x is two x plus one,

so we should make that substitution,

if we say u is equal to x squared plus x,

then we could say du dx,

the derivative of u with respect to x

is equal to two x plus one,

if we treat our differentials
like variables or numbers,

we can multiply both sides by dx,

which is a little bit of
hand wavy mathematics,

but it's appropriate here,

so we could say two x plus one times dx

and now what's really interesting here

is we have our u right over there

and notice we have our
two x plus one times dx,

in fact it's not conventional
to see an integral

rewritten the way I'm about
to write it, but I will,

I could rewrite this integral,
you should really view

this is the product of three things,

oftentimes people will just view the dx

as somehow part of the integral operator,

but you could rearrange it,

this would actually be legitimate,

you could say the integral
of the square root

of x squared plus x
times two x plus one dx

and if you wanted to be really clear,

you could even put all of those things

in parentheses or something like that

and so here,

this is our u

and this right over here is our du

and so we could rewrite this as being

equal to the integral

of the square root of u,

'cause x squared plus x is u,

times du, which is much
easier to evaluate.

If you are still confused there,

you might recognize it if I rewrite this

as u to the one half power,

because now we could just
use the reverse power rule

to evaluate this

and then we would have
to undo the substitution,

once we figure out what
this anti derivative is,

we then would then reverse substitute

the x expression back in for the u.