For example, if f (x) = x3, then f ′(x) = 3x2 and so
f ′(0) = 0 But 0 is neither a point of local maxima nor
a point of local minima (Fig 6 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