13) ANote   A point c in the domain of a function f at
which either f ′(c) = 0 or f is not differentiable is
called a critical point of f Note that if f is continuous
at c and f ′(c) = 0, then there exists an h > 0 such
that f is differentiable in the interval
(c – h, c + h) We shall now give a working rule for finding points of local maxima or points of
local minima using only the first order derivatives