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 th