Optimization finding expression

[Hollysmoke]

http://img117.imageshack.us/img117/3055/diagram28do.jpg

I drew out that diagram but I think I might be wrong. The question is:

Tom makes an open box from a rectangular piece of metal by cutting equal squares from the four corners and turning up the sides. The piece of metal measures 60 x 100 cm. What are the dimensions of the box with the maximum volume?

 

[Hollysmoke]

I wrote A=(60-2x)(100-2y)

 

[PPonte]

$$V = l.w.h$$
$$V = (100-2y)(60-2x)(y)$$

If he cut squares from the four corners, then y = x.

$$V = (100-2x)(60-2x)(x)$$

Find the maximum value of V for positive values of x and you have your solution.

 

[Hollysmoke]

OHH...Right. Okay, thank you!

 

[Hollysmoke]

Okay this is strange...
After expanding and simplifying, I get
V=4x^3-320x^2+6000x

I differentiate that too
V'=12x^2-640x+6000

but I'm not getting the answer in the textbook. What am I doing wrong?

 

[Hollysmoke]

nvm...I should have differentiated using the product rule D8

 

[Hollysmoke]

no...didn't make a different and I am still getting a weird answer. Can someone please help me?

 

[Hollysmoke]

Ignore last post! D8