Class CBSE Class 12 Mathematics Applications of Derivatives Q #1253
KNOWLEDGE BASED
APPLY
3 Marks 2024 AISSCE(Board Exam) SA
Find the intervals in which the function $f(x)=\frac{log~x}{x}$ is strictly increasing or strictly decreasing.

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Detailed Solution

Step 1: Find the derivative of f(x)

Given the function $f(x) = \frac{\log x}{x}$, we need to find its derivative $f'(x)$ using the quotient rule. The quotient rule states that if $f(x) = \frac{u(x)}{v(x)}$, then $f'(x) = \frac{u'(x)v(x) - u(x)v'(x)}{[v(x)]^2}$.

Step 2: Apply the quotient rule

Here, $u(x) = \log x$ and $v(x) = x$. Thus, $u'(x) = \frac{1}{x}$ and $v'(x) = 1$. Applying the quotient rule, we get: $$f'(x) = \frac{\frac{1}{x} \cdot x - \log x \cdot 1}{x^2} = \frac{1 - \log x}{x^2}$$

Step 3: Determine the critical points

To find the intervals where $f(x)$ is increasing or decreasing, we need to find the critical points by setting $f'(x) = 0$. $$\frac{1 - \log x}{x^2} = 0$$ This implies $1 - \log x = 0$, so $\log x = 1$. Therefore, $x = e$ (since we are considering the natural logarithm).

Step 4: Analyze the sign of f'(x)

We need to analyze the sign of $f'(x)$ in the intervals $(0, e)$ and $(e, \infty)$. Note that the domain of $f(x)$ is $x > 0$. - For $0 < x < e$, $\log x < 1$, so $1 - \log x > 0$. Since $x^2 > 0$, $f'(x) = \frac{1 - \log x}{x^2} > 0$. Thus, $f(x)$ is strictly increasing in the interval $(0, e)$. - For $x > e$, $\log x > 1$, so $1 - \log x < 0$. Since $x^2 > 0$, $f'(x) = \frac{1 - \log x}{x^2} < 0$. Thus, $f(x)$ is strictly decreasing in the interval $(e, \infty)$.

Final Answer: f(x) is strictly increasing in (0, e) and strictly decreasing in (e, ∞)

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student needs to apply the concepts of differentiation and increasing/decreasing functions to solve the problem. They must use the quotient rule to find the derivative and then analyze the sign of the derivative to determine the intervals of increase and decrease.
Knowledge Dimension: PROCEDURAL
Justification: The question requires the student to apply a specific procedure (differentiation using the quotient rule, finding critical points, and analyzing the sign of the derivative) to determine the intervals of increasing and decreasing behavior of the function.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE. The question directly tests the student's understanding of the application of derivatives to find intervals of increasing and decreasing functions, a standard topic in the syllabus.