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Quadratic equation
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For ax^2 + bx + c = 0 with a ≠ 0, let the discriminant be D = b^2 − 4ac.

If D > 0 there are two distinct real roots x = (−b ± √D) / (2a);
if D = 0 there is one real double root x = −b / (2a);
if D < 0 there are two complex conjugate roots x = (−b ± i√(−D)) / (2a).

A quick check is to substitute each root back into ax^2 + bx + c and
verify the residual is (numerically) 0.
