How can I find the limit for [f(x)-cosa]/(x-a) using l'Hopital's rule? Note: when x≠a, f(x)= [sinx-sina]/ (x-a) when x=a, f(x)= cosa So,here's what I know, Since f(x)= cosa, then f(a)= cosa and therefore, substituting this into [f(x)-cosa]/(x-a) gives [f(x)-f(a)]/(x-a) l'Hopital's rule says that to find the limit, we can differentiate the numerator and denominator seperately. How do I do that? Is it like this? for the numerator =>[f'(x) - f'(a)] for the denominator, should I differentiate it with respect to x or a?? I don't know how to differentiate x-a. Help!