Can anyone help me with this question???

prove f(x)=||x||^2 is differentiable using the definition of differentiability

where x is a vector in R^n. and ||x|| is the length of the vecotr(Not allowed to use use components of x)

x=(x1,x2,...xn)

The definitoin is lim as h->0, [f(a+h)-f(a) -c*h]/h =0

where c is the gradient of f,the function is differentiable at point a

I have absolutely no idea where to start, it's not like you have two distinct vairables..x,y....and prove differentiable at a particular point, and I am not allowed to use the components of x.How do i prove it's differentiable everywhere...

Anyone give me some hints?

Thanks so much