de Moivre's formula

Etymology
Named after (1667–1754), French mathematician.

Proper noun

 * 1)  A formula that connects trigonometry and complex numbers, stating that, for any complex number (and, in particular, for any real number) x and integer n, $$\big(\cos (x) + i \sin (x)\big)^n = \cos (nx) + i \sin (nx),$$ where i is the imaginary unit.