Find portion of Earth between line of longitude.

Question

Find the portion of the Earth between 10 degrees west and 40 degrees west.

I am not sure how to approach this problem. I found the radii of the circles created by the longitudinal lines, but I can not find the surface area of the spherical cap.

Answer 1

hint

All the Earth corresponds to $ 360 ^° $, and your portion to $ 40-10=30^°=\frac{360}{12}$