Find the maximum value of 2x3 – 24x + 107 in the interval [1, 3]. Find the maximum value of the same function in [–3, –1].
Let f (x) = 2x3 – 24x + 107, x ∈ [1, 3]
⇒ f’(x) = 6x2 – 24
=6(x2 - 4)
Now, f’(x) = 0
⇒ 6(x2 - 4) = 0
⇒ x2 = 4
⇒ x = �2
Therefore, we will only consider the interval [1,3]
Now, we evaluate the value of f at critical point x = 2 ϵ [1,3] and at end points of the interval [1,3].
f(2) = 2(2)3 – 24(2) + 107
= 2(8) – 24(2) + 107
= 75
f(1) = 2(1)3 – 24(1)+ 107
= 2 – 24 +107
= 85
f(3) = 2(3)3 – 24(3) + 107
= 2(27) -24(3) +107
= 89
Therefore, we have the absolute maximum value of f on [1,3] is 89 occurring at x =3.
Now, we will only consider the interval [-3, -1]
Now, we evaluate the value of f at critical point x = -2 ϵ [-3, -1] and at end points of the interval [1,3].
f(-3) = 2(-3)3 – 24(-3) + 107
= 2(-27) – 24(-3) + 107
= 125
f(-1) = 2(-1)3 – 24(-1)+ 107
= -2 + 24 +107
= 129
f(-2) = 2(-2)3 – 24(-2) + 107
= 2(-8) -24(-2) +107
= 139
Therefore, we have the absolute maximum value of f on [-3, -1] is 89 occurring at x = -2.