作代换(1-x)/(1+x)=t1-x=t+txx=(1-t)/(t+1)即f(t)=(1-t)/(t+1)t换成xf(x)=(1-x)/(x+1)
(1-x)/(1+x)=t 1-x=t+tx x=(1-t)/(t+1) 即f(t)=(1-t)/(t+1)
