# Homework Help: Volume of a box using differentiation

1. Jul 19, 2009

### Matty R

Hello

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

1. The problem statement, all variables and given/known data
http://img268.imageshack.us/img268/2161/19947313.jpg [Broken]

2. Relevant equations
http://img139.imageshack.us/img139/3478/24591171.jpg [Broken]

3. The attempt at a solution
http://img187.imageshack.us/img187/2808/54885491.jpg [Broken]

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.

Last edited by a moderator: May 4, 2017
2. Jul 19, 2009

### 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.

3. Jul 19, 2009

### Matty R

A = 4ab + b^2.

I fall for stuff like that all the time.

Thank you very much.