We want to make a tank, in the shape of a rectangular box with open top, to contain
36 m3. The length of the tank must be twice its width. Find the dimensions of the
tank that minimise its surface area. In your answer book, explain your steps, and
write your nal results in the form w =?; l =?; h =? for the width w, length l and
height h. Hint: express the surface area as a function of width, take the derivative
and set it to zero.