In fact, it is Vedic Theorem

x1+x2=-b/a

The proof process is the proof process of Vedic Theorem,

The formula for finding the root of a quadratic equation of one variable is:

X1= (-b+√b^2-4ac)/2a, X2= (-b-√b^2-4ac)/2a　

X1﹢X2= (-ab+ 2a-4ac)/2a+（-b-√b^2-4ac)/2a　

So X1﹢X2=-b/a

X1X2= [（-b+√b ^2-4ac﹚÷2a]×[（-b-√b^2-4ac﹚÷2a]　

So X1X2=c/a