CBSE Class 12 Mathematics Applications of Derivatives Q #608
KNOWLEDGE BASED
APPLY
1 Marks 2024 AISSCE(Board Exam) MCQ SINGLE
The function \(f(x)=\frac{x}{2}+\frac{2}{x}\) has a local minima at x equal to:
(A) 2
(B) 1
(C) 0
(D) -2

AI Tutor Explanation

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Step-by-Step Solution

  1. Step 1: Find the first derivative of the function.

    Given \(f(x) = \frac{x}{2} + \frac{2}{x}\), we find its derivative \(f'(x)\):

    \(f'(x) = \frac{1}{2} - \frac{2}{x^2}\)

  2. Step 2: Find the critical points by setting the first derivative equal to zero.

    \(\frac{1}{2} - \frac{2}{x^2} = 0\)

    \(\frac{1}{2} = \frac{2}{x^2}\)

    \(x^2 = 4\)

    \(x = \pm 2\)

  3. Step 3: Find the second derivative of the function.

    \(f''(x) = \frac{d}{dx} (\frac{1}{2} - \frac{2}{x^2}) = \frac{4}{x^3}\)

  4. Step 4: Apply the second derivative test to determine local minima.

    For \(x = 2\), \(f''(2) = \frac{4}{2^3} = \frac{4}{8} = \frac{1}{2} > 0\). Since the second derivative is positive, \(x = 2\) is a local minima.

    For \(x = -2\), \(f''(-2) = \frac{4}{(-2)^3} = \frac{4}{-8} = -\frac{1}{2} < 0\). Since the second derivative is negative, \(x = -2\) is a local maxima.

  5. Step 5: Identify the x value where the function has a local minima.

    The function has a local minima at \(x = 2\).

Correct Answer: 2

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student needs to apply the concepts of derivatives and local minima to solve the given problem. They must differentiate the function, find critical points, and then use the second derivative test to determine the local minima.
Knowledge Dimension: PROCEDURAL
Justification: The question requires a series of steps (differentiation, finding critical points, applying the second derivative test) to arrive at the solution. This involves knowing the procedures for finding local minima/maxima.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE. The question directly tests the student's understanding and application of standard calculus techniques covered in the textbook.

Core Concept

Topic: Second Derivative Test for Local Minima


Rule: To find the local minima of a function $f(x)$, we use the second derivative test.



  • Step 1: Find critical points by setting $f'(x) = 0$.

  • Step 2: Find the second derivative $f''(x)$.

  • Step 3: Substitute the critical point ($c$) into $f''(x)$.



Key Formula:
$$\text{If } f''(c) > 0 \Rightarrow x = c \text{ is a point of Local Minima}$$
$$\text{If } f''(c) < 0 \Rightarrow x = c \text{ is a point of Local Maxima}$$

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