Question
A square tank of capacity 250 cubic metres has to be dug out. The cost of the land is Rs.50 per square metre. The cost of digging increases with the depth and for the whole tank is Rs 400 h2, where h metres is the depth of the tank. What should be the dimensions of the tank so that the cost be minimum?
Solution
Let x side of square base.
Volume of tank = 250 cubic metres.
...(1)
Cost of land = 
Cost of digging = 
Let C be total cost.
When 
