Just checking this now :\ I had to show that f is not differentiable at 0 so:

Potential Proof:

I think I have made a big booboo and gone completely wrong, here is the whole of the question (its not an exam question, but a simple question from a group that our lecturer has given us to try and do over summer before anyone asks)

PS: It is spose to show f(h) over the modulus of h and in the line below, h over modulus of h