In Exercises 55 and 56, use the Second Theorem of Pappus, which is stated as follows:
If a segment of a plane curve C is revolved about an axis that does not intersect the curve (except possibly at its endpoints), then the area S' of the resulting surface of revolution is equal to the product of the length of C times the distance d traveled by the centroid of C.
A torus is formed by revolving the graph of about the y-axis. Find the surface area of the torus.
