Three point charges of equal value q are placed at the vertices of al equilateral triangle. The resulting lines of force should be sketch as in

When Q is at position x, thenU=qQ4πε01(a+x)+1(a−x)   =2aqQ4πε0a2−x2∴ dUdx=2aqQ4πε02xa2−x22=2aqQ4πε0×2xa4∵a2−x2=a2 when x is small or   dU∝xdx or U∝x2.