00:04
Okay, here's a question that gives us a function, negative x squared plus bx minus 75, and a max value of 25 of the function, not x equal to 25, that's f of x equals 25.
00:14
And it asks us to find a value of b, such that the function has that given maximum.
00:20
So recall that the function of maximum occurs at the point where b, where x is equal to negative b over 2a.
00:26
So let's figure out what negative b over 2a is here.
00:29
So a is negative 1 here.
00:32
So that would be, let's write this off to the side.
00:36
Negative b over 2 times negative 1.
00:39
It's going to be b over 2, right? so for our case, the function maximum is going to occur at b over 2.
00:49
So now all we need to do is plug that in for x, right? so f of b over 2 is going to be negative b squared over 4 plus b times b over 2, which i'll write as b squared over 2 minus 75.
01:09
And we know that the max value is 25...