Euler's formula - Wikipedia
https://en.m.wikipedia.org/wiki/Euler%27s_formula
Around 1740 Leonhard Euler turned his attention to the exponential function and derived the equation named after him by comparing the series expansions of the exponential and trigonometric expressions.[6][4] The formula was first published in 1748 in his foundational work Introductio in analysin infinitorum.[7]