(Euler's theorem) In $\triangle{ABC}$, let $R$ and $r$ be its circumradius and inradius, respectively, show that $$|OI|^2 = R^2 - 2Rr$$ where $O$ is the circumcenter and $I$ is the incenter. This relation can also be rewritten as $$\frac{1}{R-d}+\frac{1}{R+d}=\frac{1}{r}$$