The number $\pi$ can be expressed as $\pi=\lim_{n\to\infty} \frac{n\sqrt[n]{-1}-n}{\sqrt{-1}}$ or more poetically $\pi=\frac{\infty\sqrt[\infty]{-1}-\infty}{\sqrt{-1}}$. Here we choose the principal branch of the root. Is there a proof using only roots, without resorting to sines and cosines? (Note: Use of the hyperreals is not required.) Does this formula have historical sources? Feel free to use l'Hopital's rule and Taylor, too.