Cardano’s derivation of the cubic formula
To solve the cubic polynomial equation for , the first step is to apply the Tchirnhaus transformation . This reduces the equation to , where
The next step is to substitute , to obtain
(1) |
or, with the terms collected,
(2) |
From equation (2), we see that if and are chosen so that and , then will satisfy equation (1), and the cubic equation will be solved!
There remains the matter of solving and for and . From the second equation, we get , and substituting this into the first equation yields
which is a quadratic equation in . Solving for using the quadratic formula, we get
Using these values for and , you can back–substitute , , , and to get the expression for the first root in the cubic formula. The second and third roots and are obtained by performing synthetic division using , and using the quadratic formula on the remaining quadratic factor.