x^2+b/ax+(b/2a)^2-（b/2a)^2+c/a=0
x^2+b/ax+(b/2a)^2=（b/2a)^2-c/a
(x+b/2a)^2=b^4/4a^2-c/a=(b^2-4ac)/4a^2
x+b/2a=±√(b^2-4ac)/2a
x=[-b±√(b^2-4ac)]/2a