13. Consider the function f: R+ -> R defined by f(x) = √x. Using the ε- definition of a two-sided limit, show that lim(x->0) f(x) = 0.
14. Let f be a real-valued function and c be an interior point in the domain of f. Write a formal definition of lim(x->c+) f(x) = L, where L diverges to infinity from the right.