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High school math thinks the answer is strange. Let's help, because we should find the right answer.
F(x)=( 1+x)/( 1-x), and the domain is A={x|x≠ 1}.

To make f[f(x)] meaningful, then f(x)≠ 1, x≠ 1.

f[f(x)]=[ 1+f(x)]/[ 1-f(x)]=[ 1+( 1+x)/[

So x≠0, from f(x)≠ 1 get x≠- 1.

So B={x|x≠0 and x≠ 1 and x≦- 1}

So b is a subset of a, so choose 2a ∩ b = b