The circle x = a cost, y = a sin(t), 0 ≤ t ≤ 2π is revolved about the line x = b, 0 < a < b, thus generating a torus (doughnut). Find its surface area.

Area of the torus:

The formula I found for this is :

S= π^2(R+r)(R-r)^2

But what is R and r ? What do I need to do to find those?