Potential created by a cone portion - electrostatic potential - Corrected exercises electromagnetism

Consider a portion of a cone ,of half-angle with the vertex a and  limit rays R1 and R2 (R1 <R2).
This system is loaded on the surface with non-uniform density:

a is a constant homogeneous to a length and r the radius of the cone at a point on its axis of symmetry.
Determine the electrostatic potential  at the vertex O of the cone.