Volume of a box using differentiation

[Matty R]

Hello

I'm really stuck on a question and could do with some help.

Homework Statement

http://img268.imageshack.us/img268/2161/19947313.jpg

Homework Equations

http://img139.imageshack.us/img139/3478/24591171.jpg

The Attempt at a Solution

http://img187.imageshack.us/img187/2808/54885491.jpg

I'm so confused. I'm going round and round in circles. I've proven that 256 does indeed equal 256, and 1280, funnily enough, equals 1280. I've tried applying the Product Rule at various stages to no avail. I just can't see how to get a numeric value of a or b.

The only thing like this that we've done in class was a question where we were given an amount of fencing and had to work out the maximum quadrilateral area that it could enclose. But I'm ending up with products equalling 0 instead of a number and a letter being added and don't know what to do.

I'd appreciate any and all help/hints/tips.

Thanks.

 

[jamesrc]

Double check your equation for the Area - not all of the 5 faces of what will be the open box have the same area. I hope that helps.

 

[Matty R]

A = 4ab + b^2.

I fall for stuff like that all the time.

Thank you very much.