00:01
Okay, so we'd like to inscribe a rectangle inside a semicircle of radius 3.
00:11
So that has equation square to 9 minus x squared.
00:18
And then we'll put a rectangle inside here.
00:25
This will be x minus x.
00:29
The area of the rectangle has a function of x.
00:33
Well, it's going to be the base, which is 2x.
00:36
And the height, which is y, or square root of 9 minus x squared, and the domain here will be x between 0 and 3.
00:46
So when x is 0, the rectangle is flat like this, when x is 3, the rectangle is flat like this.
00:54
So a 0 is 0, and a of 3 is 0.
01:00
So we know that the maximum is going to occur somewhere in the interior of this interval, so at a critical point inside of here.
01:08
So if we take the derivative, we'll get 2 times root 9 minus x squared, and then minus 2x, we'll get a 2x by the chain rule, so 4x squared over 2 root 9 minus x squared.
01:30
And so let me multiply the top and bottom here by 2 root 9 minus x squared to get a common denominator so we have a prime is four times nine minus x squared minus 4x squared all over 2 root 9 minus x squared okay looks good we can cancel one factor of 2 okay and so we just want to know when does a prime equals 0 well that's going to be when it's going to be 18 minus 2 x squared so i'm just saying the numerator to be 0...