Target Exam

CUET

Subject

-- Mathematics - Section A

Chapter

Applications of Derivatives

Question:

Function $f(x) = x^3 − 3x + 3$ is

(A) Increasing in the interval (-1, 1)
(B) Increasing in the interval (1, ∞)
(C) Decreasing in the interval (-1, 1)
(D) Increasing in the interval (-∞, -1)∪(1,∞)

Choose the correct answer from the options given below:

Options:

(A), (C) and (D) only

(B), (C) and (D) only

(A) and (B) only

(A), (B) and (D) only

Correct Answer:

(B), (C) and (D) only

Explanation:

The correct answer is Option (2) → (B), (C) and (D) only

Given function:

\(f(x) = x^3 - 3x + 3\)

Find the derivative:

\[ f'(x) = 3x^2 - 3 = 3(x^2 - 1) \]

Factor derivative:

\[ f'(x) = 3(x - 1)(x + 1) \]

Set \(f'(x) = 0\) to find critical points:

\[ 3(x - 1)(x + 1) = 0 => x = -1, 1 \]

Check the behavior of \(f'(x)\) around critical points by evaluating the sign:

IntervalTest point\(f'(x)\) signBehavior of \(f(x)\)
\((-\infty, -1)\)\(-2\)\(f'(-2) = 3(4 - 1) = 9 > 0\)Increasing
\((-1, 1)\)\(0\)\(f'(0) = 3(0 - 1) = -3 < 0\)Decreasing
\((1, \infty)\)\(2\)\(f'(2) = 3(4 - 1) = 9 > 0\)Increasing

Conclusion:

  • Function is increasing on \((-\infty, -1)\) and \((1, \infty)\)
  • Function is decreasing on \((-1, 1)\)

Thus, correct options are:

(B), (C), and (D)