Question 26

Mathematics Differentiation Hard

A condition that x<sup>3</sup> + ax<sup>2</sup> + bx + c may have no extremum is

(A) a<sup>2</sup>&nbsp;&ge; 3b
(B) b<span style="position: relative; line-height: 0; vertical-align: baseline; top: -0.5em;font-size:10.5px;">2</span>&nbsp;&lt; 3b
(C) a<span style="position: relative; line-height: 0; vertical-align: baseline; top: -0.5em;font-size:10.5px;">2</span>&nbsp;&lt; 3b
(D) b<span style="position: relative; line-height: 0; vertical-align: baseline; top: -0.5em; font-size:10.5px;">2</span>&nbsp;&ge; 3b
View Dynamic Solution & Explanation
Correct Solution: Option C

Step-by-step Solution:

To determine the conditions under which the function \( f(x) = x^3 + ax^2 + bx + c \) has no extremum, we can proceed with the following steps: 1. Find the First Derivative: \[ f'(x) = \frac{d}{dx} (x^3 + ax^2 + bx + c) = 3x^2 + 2ax + b \] 2. Condition for No Extremum: For the function to have no extremum, the first derivative must be positive for all \( x \): \[ 3x^2 + 2ax + b > 0 \] 3. Quadratic Inequality: The quadratic expression \( 3x^2 + 2ax + b \) will be positive for all \( x \) if: - The coefficient of \( x^2 \) is positive (\( 3 > 0 \)), which is always true. - The discriminant \( D \) is negative: \[ D = (2a)^2 - 4 \times 3 \times b < 0 \] \[ 4a^2 - 12b < 0 \] \[ a^2 - 3b < 0 \] \[ a^2 < 3b \] Therefore, the condition for the function \( f(x) = x^3 + ax^2 + bx + c \) to have no extremum is: \[ \boxed{a^2 < 3b} \]