Sum of Roots of Quadratic Equation

Theorem
Let $P$ be the quadratic equation $a x^2 + b x + c = 0$.

Let $\alpha$ and $\beta$ be the roots of $P$.

Then:
 * $\alpha + \beta = -\dfrac b a$

Also see

 * Product of Roots of Quadratic Equation
 * Viète's Formulas