GRE Conservation of Energy problem.

[PsychonautQQ]

Homework Statement

http://grephysics.net/ans/9277/87

Can somebody explain to me why the electric potential = -K/2r^2?
If you integrate Fdr and F = K/r^3, where does the 2 in the denominator come from?

 

[haruspex]

Homework Statement

http://grephysics.net/ans/9277/87

Can somebody explain to me why the electric potential = -K/2r^2?
If you integrate Fdr and F = K/r^3, where does the 2 in the denominator come from?

Standard rules. What's the integral of xⁿ.dx? Now put n = -3.