1. The problem statement, all variables and given/known data Hi all, I am working through Gravity by James Hartle and have become stuck on a question asking me to calculate the area of a circle of radius r in the 2D geometry that is the surface of a sphere of radius a. A surface element on this sphere can be found to be (adΘ)(asinΘdΦ) (see attached image). 2. Relevant equations 3. The attempt at a solution Integrating the above surface element over Θ=0 to Θ=r/a, and Φ=0 to Φ=2π, I obtain A=-2πa2cos(r/a). The answer given in the solutions manual is this, plus 2πa2. I don't understand how the additional 2πa2 comes about.