Don't forget that the length is twice the width. That is, .
So:
Take the derivative from here.
A rectangular container with no top has a volume of 20 m^3. The length of its base is twice the width.
The cost for the material of the base is $5 per m^2.
The cost for the material of the sides are $12 per m^2.
What is the cost for the cheapest container?
My attempted work:
Let x be the width. Let y = length.
V = 20 = xyz.
C = 5xy + 12(2xz) + 12(2yz)
C = 5xy + 24xz + 24yz
V = 20 = xyz
z = 20/(xy)
C = 5xy + 240/x + 240/y
Cx = 5y - 240/(x^2)
Cy = 5x - 240/(y^2)
Not sure how to get the answer. Can somebody show me how?