Use the definition to find the derivative of f(x)= , for >0. Is f differential at zero? Explain.

I believe the def we need is let f: D->R with an accumulation point of D and D.with x not equal to x_0, define T(x)=f(x)-f(x_0)/x-x_0. The function f is said to be differentiable at x_0 iff T has a limit at x_0 and we write lim_x->x_0 T(x)=f'(x_0). ..I understand it I'm just sure how to show a proof and stuff..