Certain formulas I really enjoy looking at like the [Euler-Maclaurin formula](https://en.wikipedia.org/wiki/Euler_Maclaurin) or the [Leibniz integral rule](https://en.wikipedia.org/wiki/Leibniz_integral_rule). What's your favorite equation, formula, identity or inequality?



What's your favorite equation, formula, identity or inequality?
$e^{\pi i} + 1 = 0$


