f（x）+2f（-x）=x²+2x (1)令x=-xf（-x）+2f（x）=x²-2x (2)(2)×2-(1)3f(x)=x²-6xf(x)=(x²-6x)/3
f(X)+2f(-x)=x2+2x;f(-X)+2f（-x)=(-x)2+-2x联立可以得到f（x)=(x2-6x)/3
