f(x)=(x-1)/(x+1),把x=1/x代入得f(1/x)=(1/x-1)/(1/x+1)=(1-x)/(1+x)两式相加得f(x)+f(1/x)＝0
f(1/x)=(1/x-1)/(1/x+1)=(1-x)/(1+x)f(x)+f(1/x)=(x-1)/(x+1)+(1-x)/(1+x)=0
