Show that the Newtonian gravitational potential near the earth due to the Moon can be expressed as = GM [x2 + y2 + (a z)2]1/2 , where M is the mass of the moon, a is the distance between the center of the earth and the center of the moon, and (x, y, z) is a Cartesian coordinate system with origin at the center of the earth and z-axis pointing straight at the center of the moon