Question

What will be the maxima for the function f(x) = x^4 –8x^3 + 22x^2 –24x + 8?

(a) 0

(b) 1

(c) 2

(d) 3

This question was addressed to me in my homework.

This interesting question is from Calculus Application topic in division Application of Calculus of Mathematics – Class 12

Answer 1

Right choice is (c) 2

Easiest explanation: We have, x^4 –8x^3 + 22x^2 –24x + 8 ……….(1)

Differentiating both sides of (1) with respect to x, we get,

f’(x) = 4x^3 – 24x^2 + 44x – 24 and f”(x) = 12x^2 – 48x + 44  ……….(2)

At an extremum of f(x), we have f’(x) = 0

Or 4x^3 – 24x^2 + 44x – 24 = 0

Or x^2(x – 1) – 5x(x – 1) + 6(x – 1) = 0

Or (x – 1)(x^2 – 5x + 6) = 0

Or (x – 1)(x – 2)(x – 3) = 0

So, x = 1, 2, 3

Now, f”(x) = 12x^2 – 48x + 44

f”(1) = 8 > 0

f”(2) = -4 < 0

f”(3) = 8 < 0

So, f(x) has maximum at x = 2.