X 2 is always greater than 0, and X 2- 1 is always less than 0, so the sign of f'(x) remains unchanged.

Because f'(x)=0 has three solutions x=- 1, x=0, x= 1.

And f(x) has only two extreme points, so we should exclude one, so we should explain that x=0 is not an extreme point.