It is given that at $x = 1$, the function $x^4 - 62x^2 + ax + 9$ attains its maximum value in the interval [0,2]. Find the value of a.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 120
Let $f(x) = x^4- 62x^2+ax +9$ ...(i)
It is differentiable for all x in [0, 2].
Differentiating (i) w.r.t. x, we get
$f'(x) = 4x^3-62.2x + a = 4x^3 - 124x + a$
$∴ f'(1) = 4.1^3 – 124.1 + a = a − 120$.
Given that at $x = 1$, the function (i) has maximum value, therefore, $x = 1$ is a point of maxima
$⇒ x = 1$ is a critical point
$⇒ f'(1)=0⇒a-120=0⇒ a = 120$.