ax1²+bx1+c=ax2²+bx2+c
a(x1+x2)(x1-x2)+b(x1-x2)=0
所以a(x1+x2)+b=0
x1+x2=-b/a
所以x=x1+x2
y=a(-b/a)²+b(-b/a)+c
=b²/a-b²/a+c
=c