f(x+4)=f(x+2+2)=1-f(x+2)/1+f(x+2) 把f(x+2)=1-f(x)/1+f(x)代进去 =[1-（1-f(x)/1+f(x)）]/[1+(1-f(x)/1+f(x))] 通分化简 =2f(x)/2 = f(x)T=4
