=(x1+x2平方-4x1x2
(x1-x2)^2=(x1)^2-2(x1)(x2)+(x2)^2=(x1)^2+2(x1)(x2)+(x2)^2-4(x1)(x2)=(x1+x2)^2-4(x1)(x2)=(-b/a)^2-4c/a=b^2/a^2-4ac/a^2=(b^2-4ac)/a^2
