Def: Let f be a real-valued function defined on an interval I containing the point c, (we allow the possibilty that c is an endpoint of I) we say that f is differentiable at c (or has a derivative at c) if the limit

lim x->c (f(x) - f(c))/(x-c)

exists and is finite.

use the definition above to find the derivative of each function.

C) f(x) = 1/x for x is not equal to 0

D) f(x) = sqrt(x) for x>0