f(x)=ax2+bx+c
f(x1)=f(x2)
则x1、x2关于对称轴x=-b/(2a)对称
x-x1=x2-x
x1+x2=2x=-b/a
f(x1+x2) = f(-b/a) = a*(-b/a)^2 + b*(-b/a) + c = b^2/a-b^2/a+c = c