Definite integrals: reverse power rule | AP Calculus AB | Khan Academy

- [Instructor] Let's evaluate
the definite integral

from negative three to five

of four dx.

What is this going to be equal to?

And I encourage you to pause the video

and try to figure it out on your own.

All right, so in order to evaluate this,

we need to remember the
fundamental theorem of calculus,

which connects the notion

of a definite integral and antiderivative.

So the fundamental theorem
of calculus tells us

that our definite integral
from a to b of f of x dx

is going to be equal to the antiderivative

of our function f, which we
denote with the capital F,

evaluated at the upper bound,

minus

our antiderivative,

evaluated at the lower bound.

So we just have to do
that right over here.

So this is going to be equal to,

well, what is the antiderivative of four?

Well, you might immediately say,

well, that's just going to be four x.

You could even think of it in
terms of reverse power rule.

Four is the same thing
as four x to the zero.

So you increase zero by one.

So it's going to be four x to the first,

and then you divide by that new exponent.

Four x to the first divided by one,

well, that's just going to be four x.

So the antiderivative is four x.

This is, you could say,
our capital F of x,

and we're going to evaluate that

at five and at negative three.

We're gonna find the
difference between these two.

So what we have right over here,

evaluating the antiderivative
at our upper bound,

that is going to be four times five.

And then from that,
we're going to subtract,

evaluating our antiderivative
at the lower bound.

So that's four times negative three.

Four times negative three.

And what is that going to be equal to?

So this is

20 and then minus negative 12.

So this is going to be plus 12,

which is going to be equal to 32.

Let's do another example

where we're going to do
the reverse power rule.

So let's say

that we want to find

the indefinite or we want to
find the definite integral

going from negative one

to three

of seven

x squared

dx.

What is this going to be equal to?

Well, what we want to do is

evaluate what is the
antiderivative of this?

Or you could say what is,
if this is lowercase f of x,

what is capital F of x?

Well, the reverse power rule,

we increase this exponent by one.

So we're going to have
seven times x to the third,

and then we divide by
that increased exponent.

So seven x to the third divided by three,

and we want to evaluate
that at our upper bound

and then subtract from that

and it evaluate it at our lower bound.

So this is going to be equal to,

so evaluating it at our upper bound,

it's going to be seven
times three to the third.

I'll just write that three
to the third over three.

And then from that, we
are going to subtract

this capital F of x,

the antiderivative evaluated
at the lower bound.

So that is going to be

seven times negative one to the third,

all of that over three.

And so this first expression, let's see,

this is going to be seven

times three to the third over three.

This is 27

over three.

This is going to be the same
thing as seven times nine.

So this is going to be 63.

And this over here,

negative one to the third
power is negative one.

But then we were subtracting a negative,

so this is just gonna be adding.

And so this is just going
to be plus seven over three.

Plus seven

over three,

if we wanted to express
this as a mixed number,

seven over three is the
same thing as 2 1/3.

So when we add everything together,

we are going to get 65 1/3.

And we are done.