Quadratic formula

Solving quadratic equations $ax^2+bx+c=0$ can be done either by factoring (if you can see the factors easily) or by using the quadratic formula, which gives the roots as

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

If it turns out that the discriminant ($b^2-4ac$) is negative, then the roots will be complex numbers.