00:01
All right.
00:02
We want to try to find a function of x and y where the partial of f with respect to x is positive and the partial of f with respect to y is negative.
00:13
If we want the partial of f with respect to x to be positive, we can start by saying f of x y equals 5 times x, 5x.
00:31
We're not done.
00:32
This is just part of our 4.
00:34
Function x of x and y.
00:36
If we took the partial of this function f with respect to x, the derivative of 5x with respect to x would be 5, that's positive, so that's greater than zero.
00:46
We're good.
00:47
But now we need to also incorporate the variable y and such that the partial of f with respect to y will be negative, less to zero.
00:59
So if i put minus 5y, subtract 5y, the derivative of negative 5y, the derivative of negative 5 y with respect to y is negative 5.
01:15
So that would be negative.
01:18
But what about this? well, the derivative of 5x with respect to the variable y, this would be treated like a constant and since it doesn't have y, this would be zero.
01:31
So let's actually show these.
01:34
Okay, we're defining our function f of xy to be 5x minus 5y...