f(x) + 2f(1/x) = x(1)
f(1/x) + 2f(x) = 1/x
2f(1/x) + 4f(x) = 2/x(2)
(2) - (1)
3f(x) = 2/x - x
f(x) = (2-x^2)/(3x)(x不等于0)