You need to find the dimensions of a cylindrical can (much like a soda or soup can) so that its volume is 81 cubic inches. The sides of the can costs 50 cents per square inch, the bottom costs 65 cents per square inch and the top of the can costs 85 cents per square inch. Find the dimensions of the can that minimizes the cost. What is the minimum cost? How do you know that your answer gives a minimum?