A rectangular storage container with an open top is to have a volume of 30 cubic meters. The length of its base is twice the width. Material for the base costs 30 dollars per square meter. Material for the sides costs 7 dollars per square meter. Find the cost of materials for the cheapest such container. Find the minimum cost.

Question

A rectangular storage container with an open top is to have a volume of 30 cubic meters. The length of its base is twice the width. Material for the base costs 30 dollars per square meter. Material for the sides costs 7 dollars per square meter. Find the cost of materials for the cheapest such container.  Find the minimum cost.

anonymous · Answer

i found the equation to be 30x^2+4xh7
=30x^2+ 840/x
then you take the dirivitive which is 
c'(x)=30x-840/x^2

anonymous · Answer

then you have that equal to 0 which i did and found the 0 to be x= third square rout to 28

anonymous · Answer

I pluged that back in for the 2nd dirivitive of the equation but my answer is not right 
i got 90

anonymous · Answer

what did i do wrong

anonymous · Answer

http://www.youtube.com/watch?v=5uJnFKmxddw&list=PLOWXFrmj5bOWVu0jQOo5p2X00f4kFEhDg&index=1&feature=plpp_video