00:01
Okay, so in this exercise we have our integral i.
00:05
This is a double integral.
00:07
The endpoints of integration with respect to x are negative 9 and 0.
00:14
Then we have the endpoints of integration with respect to y.
00:19
These ones are negative square root of 91 minus x squared and 0.
00:27
Here we are integrating our function 1 over 1 plus square root of x squared plus y squared.
00:37
Okay, now before we go on, let's sketch our region of integration.
00:44
This is our cartesian plan, the x -axis, the y -axis.
00:48
Here we have our circle, x squared plus y squared less than or equal to 81.
01:01
That is a circle with center of the origin and radius 9.
01:06
Now we know that x belongs to the interval negative 9, 0.
01:10
So negative 9 here, 0 here.
01:15
Okay, y is greater than or equal to this guy here and less than or equal to 0.
01:23
So it means that our region of integration is this one.
01:30
Perfect.
01:31
Now, in polar coordinates, what are we going to have? well, an integral with respect to the angle.
01:39
This one is going to be an integral from pi to 3 pi halves.
01:46
Then an integral with respect to the radius from 0 to 9.
01:51
Our function in polar coordinates, which is 1 over 1 plus r.
01:59
And the infinitesimal element of area in polar coordinates, which is r dr d theta.
02:14
Perfect.
02:15
Okay.
02:16
Now the integral with respect to theta.
02:19
Well, this is just pi halves.
02:22
Easy.
02:22
So we are left with the integral from 0 to 9 of r over 1 plus r.
02:35
Okay.
02:37
Perfect.
02:38
Okay.
02:39
We just need to compute this integral here.
02:45
Okay, let's do it here.
02:47
The integral from 0 to 9 of r over 1 plus r...