To avoid using constants I will do,Originally Posted bybecky

Where,

To find the maximimumumumumum you need the critical points.

Begin by finding the derivative,

Thus,

Since, the derivative exists everywhere we need when the derivative is zero, that is,

Since divide both sides by it,

Simplify,

Thus,

Open sesame,

Multiply by 2 and divide by "A":

Thus,

Swictch signs,

Thus,

Thus,

Thus, (substitute values back inside)

Reject the negative thus,

.

(Now the trick is to show that 10 is a maximum )