Viète (1540-1603) worked on what is known as Horner's method for the approximation of polynomial roots, and also he discovered another substitution to solve cubics of the form . Horner's method for approximating roots is similar in concept to the Newton-Raphson method. Take for example a quadratic equation . Substitute where is known to be a good approximation of x. If is small, i.e. if is small, then will be very small, and may be cancelled. So we get . Proceeding inductively we can approximate any root to any degree of accuracy. Notice that is the derivative of . The new substitution for solving the cubic involved , which brought to solving , a quadratic in .