CBSE Class 12 Mathematics Applications of Derivatives Q #1758
COMPETENCY BASED
APPLY
2 Marks 2026 AISSCE(Board Exam) VSA
Determine the values of x for which $f(x)=\frac{x-3}{x+1}$, $x\ne -1$ is an increasing function.

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Detailed Solution

Step 1: Find the derivative of the function

Given the function $f(x) = \frac{x-3}{x+1}$. Using the quotient rule $\left(\frac{u}{v}\right)' = \frac{u'v - uv'}{v^2}$, where $u = x-3$ and $v = x+1$:

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

Step 2: Analyze the condition for an increasing function

A function is strictly increasing if $f'(x) > 0$. Here, $f'(x) = \frac{4}{(x+1)^2}$. Since the numerator is a positive constant ($4$) and the denominator is a square of a real number (which is always positive for $x \neq -1$), the derivative is always positive for all $x$ in the domain.

Step 3: Conclusion

Since $f'(x) > 0$ for all $x \in \mathbb{R} \setminus \{-1\}$, the function is strictly increasing on its entire domain.

Final Answer: The function is increasing for all x in its domain, i.e., x \in (-\infty, -1) \cup (-1, \infty)

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student must apply the quotient rule of differentiation and the criteria for monotonicity to determine the interval of increase.
Knowledge Dimension: PROCEDURAL
Justification: The question requires a step-by-step algorithmic process involving differentiation and inequality solving.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. It tests the conceptual understanding of the Application of Derivatives (Chapter 6) rather than rote memorization.
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