ax^2+bx+c=0
∴x^2+(b/a)x+(c/a)=0
∴(x^2+(b/a)x+(b/2a)^2)+(c/a)-(b/2a)^2=0
∴(x+b/2a)^2=(b/2a)^2-c/a
∴x+b/2a=正负√[(b/2a)^2-c/a]
∴x=-b/2a加减√[.]