A plastic rod has been bent into a circle of radius R=8.20cm. It has a charge Q1 = +4.20pC uniformly distributed along one-quarter of its circumference and a charge Q2 = -6Q1 uniformly distributed along the rest of circumference. With V=0 at infinity, what is the electric potential a) at the center of the circle and b) at point P, which is on the central axis of the central axis of the circle at distance D = 6.71cm from the center? In the problem, I tried to evaluate V = kq/r to solve this problem. I think that r and k, which is 8.99*10^9, is constant in this problem. I would like to know to integrate this equation and to make it relate with 2Pi*r P.S. Answer from textbook: a) = -2.3V and b) = -1.78V. I want to find out the solution.