Cubic equation

In mathematics, and more specifically algebra, a cubic equation is an equation involving only polynomials of the third degree. Although cubic equations occur less frequently in real-world problems than quadratic or linear equations, they still do occur.

Every cubic equation with real number coefficients has at most three real roots, as dictated by the fundamental theorem of algebra. Solving cubic equations is more difficult than solving quadratic equations, where the quadratic formula can be applied, or linear equations, which can be solved using arithmetic operations. As with quadratic equations, there is a closed formula for solving cubic equations, the cubic formula.

The cubic formula is much more complicated than the quadratic formula. Furthermore, there are examples of cubic equations with real coefficients and three distinct real solutions for which the cubic formula requires one to calculate a root of a complex number. This is more difficult than finding the root of a real number, which is all that the quadratic formula requires. It is precisely this difficulty which first demonstrated to mathematicians the utility of complex numbers.

Because the cubic formula is unwieldly, most students are taught to solve only small class of cubic equations, namely, those with at least one rational root. For such equations, all roots can be found by factoring and a possible application of the quadratic formula. When the coefficients are rational, the initial factor can easily be found using the rational root theorem.