00:08
We are told that x plus y equals 90.
00:12
And we want to maximize x squared y.
00:17
And so i'm going to write that function as p equals x squared y.
00:24
The first thing i want to do is express y in terms of x.
00:29
So i'm going to subtract x from both sides.
00:33
When i do that, i get y equals 90 minus.
00:39
X when now my p is x squared times 90 minus x and so this p is 90 squared minus x cubed now and let's go ahead and find the domain we'll write the domain in green so we know that x and are non -negative numbers.
01:24
Therefore, we know that our x is greater than or equal to zero, and we know our y is greater than or equal to zero.
01:34
Why is 90 minus x, so that means 90 minus x is greater than or equal to zero.
01:44
So when we put those together, what that tells us is that our x has to be less than or equal to 90, but we also know that our x is greater than or equal to zero.
01:55
So there is our domain.
01:58
Alright, now because we want to maximize p, let's find our derivative.
02:03
So we'll have p prime of x equals 180x minus 3x squared.
02:20
Notice i used the p that was right there and i took the derivative to find my p prime of x...