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.