The Quadratic Formula

A quadratic equation has the form $ax^2 + bx + c = 0$. The quadratic formula solves for $x$:

$x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$

The $\pm$ means two solutions exist (one with $+$, one with $-$). In competitive programming, you use this when analyzing recurrence relations or solving optimization problems with parabolic constraints.