[20 pts] In a spherical coordinate system, the axis of a charged ring has the direction 8 '+ q: 'q3 , and the center of the ring 0' is located at (d , 6 ',q) ') . The radius of the ring is a, and charge q is uniformly distributed on the ring. Write the scalar potential (I) (r, 6 , o) anywhere using spherical harmonics expansion. A datum)