The formula of x 1+x2 is Vieta's theorem, that is, x 1+x2 =-b/a, which was put forward by the French mathematician Francois Viete in his book Identification and Correction of Equations. He discovered and established the relationship between the roots and coefficients of the equation. Because David first discovered this relationship between the roots and coefficients of algebraic equations, people call this relationship the Vieta theorem.