・Inner Polygon : $\pi = n * \sin(\pi/n)$
・Outer Polygon : $\pi = n * \tan(\pi/n)$
・Leibniz : $\displaystyle \pi = \arctan{1} = 4 \sum_{n=0}^\infty \frac{(-1)^n}{2n+1} = 1 - \frac{1}{3} + \frac{1}{5} - \frac{1}{7} + \frac{1}{9} + \cdots$
・Machin : $\displaystyle \pi = 4 \left( 4 \arctan{\frac{1}{5} - \arctan{\frac{1}{239}} } \right)$