Quadratic formula
= Quadratic formula
{title2=$x=(-b\pm\sqrt{b^2-4ac})/(2a)$}
{wiki}
For a <quadratic equation> $ax^2+bx+c=0$ with $a\ne0$, completing the square gives
$$
x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}.
$$
Over the <complex numbers> both roots exist, with a repeated <root of a polynomial> when the <discriminant> $b^2-4ac$ is zero. Over the real numbers the discriminant determines whether there are two distinct roots, one repeated root, or no real root.