max/min question (1 Viewer)

[Tha Wishkah]

seem to be having a mental blank atm in my revison:

Question: a rectangular prism with square base has surface area = 250cm^2
a) Show volume
V= (125x - x^3 )
. . . . . 2

I can do this, but then
b) Find dimensions to give max volume.

I know you've got to differentiate the above equation for V, but i always find V' to be the same as V, so i'm doing something wrong and can't figure out what =\

 

[Timothy.Siu]

V= (125x - x^3 )
. . . . . 2

v'= (125-3x^2)/2
so 3x^2=125
x^2=125/3
x=root(125/3)
? looks wrong