f（1/1-x）=（1/x）f（x）+2,则f（3）=
f(3)=f[1/(1-2/3)]=(3/2)f(2/3)+2=3f(2/3)/2+2
f(2/3)=f(1/(1+1/2))=(-2)f(-1/2)+2
f(-1/2)=f(1/1-3)=(1/3)f(3)+2
接上面上个方程的到f（3）=-1/2