f(x1)=f(x2)
a(x1)^2+b(x1)=a(x2)^2+b(x2)
a(x1+x2)(x1-x2)+b(x1-x2)=0
x1≠x2,所以a(x1+x2)+b=0
f(x1+x2)=a(x1+x2)^2+b(x1+x2)=0