哥哥姐姐，叔叔阿姨，帮帮忙啊，一个数学题···

解: 令(1-x)/(1+x)=t, 解得x=(1-t)/(1+t), x²=(1-2t+t²)/(1+2t+t²)
那么有f(t)=(1-x²)/(1+x²)
=[1-(1-2t+t²)/(1+2t+t²)]/(1+(1-2t+t²)/(1+2t+t²)]
=2t/(1+t²)
所以得f(x)的解析式为f(x)=2x/(1+x²)
答案为C
祝你学习进步,如有不明可以追问.同意我的答案请采纳,O(∩_∩)O谢谢

选C，理由如下：
解：设t=(1-x)/(1+x) 则可得：x=(1-t)/(1+t)
f(t)=[1-(1-t)²/(1+t)²]/[1+(1-t)²/(1+t)²]
=[(1+t)²-(1-t)²]/[(1+t)²+(1-t)²]
=4t/(2+2t²)
=2t/(1+t²)
所以可得：f(x)=2x/(1+x²)

令1-X/1+X=Y 可得 X=1-Y/Y+1 所以F（1-X/1+X）=1-X^2/1+X^2 等价于
F（Y）={1-(1-Y)^2/(1+(1-Y)^2/(1+Y)^2}/{1+(1-Y)^2/(1+(1-Y)^2/(1+Y)^2}
=2Y/1+Y^2
所以答案为C

令y=(1-x)/(1 x),解得x=(1-y)/(1 y)，带入(1-x²)/(1 x²),f(y)=2y/(y² 1),所以选c

换元 令t=（1-x）/(1+x) 用t表示x 带回原式