f[f(x)]
=1/[1/(x+1)+1]
分母不等于0
所以x+1≠0且1/(x+1)+1≠0
x≠-1
1/(x+1)≠-1
x+1≠-1
x≠-2
所以定义域(-∞,-2)∪(-2,-1)∪(-1,+∞)