(c) constant in (a, b), if f (x) = c for all x ∈ (a, b), where c is a constant ® 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 ® First Derivative Test Let f  be a function defined on an open interval I Let
f   be continuous at a critical point c in I