Let f(x)=(x^(4))(2+sinx^(-1)), if x≠0

...........0, if x=0

Prove that f is differentiable on R (the real numbers).

Prove that f has an absolute minimum at x=0.

Prove that f ' takes on both positive and negative values in every neighborhood of 0.

Pretty lost on this problem, I would really appreciate some help please :)