f(x)=x/(1-x)f[f(x)]=[x/(1-x)]/[1-x/(1-x)] 分子分母同乘以1-xf[f(x)]=x/(1-2x)f{f[f(x)]}=[(x/(1-2x)]=[x/(1-2x)]/[1-x/(1-2x)] 分子分母同乘以1-2xf{f[f(x)]}=x/(1-3x)
