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 – 24 x + 107
∴ f'(x) = 6 x2 – 24 = 6(x2 – 4)
Now,
f'(x) = 0
⇒ 6(x2 – 4) = 0
⇒ x2 = 4
⇒ x = ±2
We first consider the interval [1, 3].
Then, we evaluate the value of f at the critical point x = 2 [1, 3] and at the end points of the interval [1, 3].
f(2) = 2 (23)– 24 (2) + 107 = 75
f(1) = 2(1)3 – 24(1) + 107 = 85
f(3) = 2(3)3– 24 (3) + 107 = 89
Hence, the absolute maximum value of f(x) in the interval [1, 3] is 89 occurring at x = 3,
Next, we consider te interval [ – 3, – 1].
Evaluate the value of f at the critical point x = – 2 [1, 3]