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