f（x）=(ax+b)/(x^2+1)是奇函数,f(-x)=-f(x)
(-ax+b)/(x^2+1)=- (ax+b)/(x^2+1),
所以b=0.
又f(1/2)=-2/5,所以(a/2)/(1/4+1)=-2/5,a=-1.
∴f(x)=-x/(x^2+1).