f(x2)-f(x1)=a(x2^2-x1^2)+2a(x2-x1)
=a(x2+x1)(x2-x1)+2a(x2-x1)
=a(1-a)(x2-x1)+2a(x2-x1)
=-a(a-3)(x2-x1)
因为00
f(x2)>f(x1)