The number f (c) is called the maximum value of f in I and the point c is called a
point of maximum value of f in I (b)
 f  is said to have a minimum value in I, if there exists a point c in I such that
f (c) < f (x), for all x ∈ I The number f (c), in this case, is called the minimum value of f in I and the point
c, in this case, is called a point of minimum value of  f  in I (c)
f is said to have an extreme value in I if there exists a point c in I such that
f (c) is either a maximum value or a minimum value of f  in I