Then
(i) f ′(c) = 0 if  f attains its absolute maximum value at c (ii) f ′(c) = 0 if  f attains its absolute minimum value at c In view of the above results, we have the following working rule for finding absolute
maximum and/or absolute minimum values of a function in a given closed interval
[a, b] Working Rule
Step 1: Find all critical points of  f  in the interval, i