Traditionally, the earth's surface has been modeled as a sphere, but the World Geodetic System of 1984 (WGS-84) uses an ellipsoid as a more accurate model. It places the center of the earth at the origin and the north pole on the positive z-axis. The distance from the center to the poles is 6356.523 km and the distance to a point on the equator is 6378.137 km.

a. Find an equation of the earth's surface as used by WGS-84.
b. Curves of equal latitude are traces in the planes z = k. What is the shape of these curves?
c. Meridians (curves of equal longitude) are traces in planes of the form y=mx. What is the shape of these meridians?