f(2)=4/5f(1/2)=(1/4)/(5/4)=1/5f(3)=9/10f(1/3)=(1/9)/(10/9)=1/10f(x)+f(1/x)=x²/(1+x²) +(1/x)²/(1 +1/x²) =x²/(1+x²) +1/(x²+1) =(x²+1)/(x²+1)=1所以 f(1)+f(2)+...