Sum of Roots of Cubic Equation

Theorem
Let $P$ be the cubic equation $a x^3 + b x^2 + c x + d = 0$.

Let $\alpha, \beta, \gamma$ be the roots of $P$.

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

Proof
From Cardano's Formula, $P$ has solutions:

where:

where:

Thus: