Thinking: f(x)=g(x)[h(x)], if H (x) = O and the value of X makes g(x)=, then f(x) is derivable at this point, which is a necessary and sufficient condition, just remember. Here [] is the absolute value.
So f (x) = (x-x-2) [x] [x+1] [x-1]
When x=0, (x-x-2 [x+1] [x-1] ≠ 0, so it is not derivable.
When x= 1, (x-x-2) [x+ 1] ≠ 0, so it is not derivable.
When x=- 1, (x-x-2) [x- 1] = 0, it can be derived.