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.