differentiable, f' not continuous

Let for and

a) Prove that is differentiable on .

b) Prove that is not continuous on .

I need help on BOTH parts. I've tried this problem, but I think I am not being very explicit in my proofs. I am also very sure my proof in b) is not nice. Is there another way to do b) [possibly without using the fundamental theorem of calculus]?

**Attempt:**

a) Clearly is differentiable at . [How do I show this?] In fact, [valid for ]. At , . Hence, is differentiable on .

b) . Let . Then because . So . So by FTC s.t. . So . But then is not a continuous function. But by FTC, is differentiable. Then , are differentiable functions whose derivatives are not continuous.

Thanks.