f(X)=1-x2/1+x2的值域
问题描述：
f(X)=1-x2/1+x2的值域
答
f(x)=-(x²+1-2)/(x²+1)
=-(x²+1)/(x²+1)+2/(x²+1)
=-1+2/(x²+1)
x²>=0
x²+1>=1
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