CBSE Class 12 Mathematics Relations and Functions Q #2143
COMPETENCY BASED
APPLY
3 Marks Generic-Curriculum SA
Let $f : R \to R$ be the function defined by $f(x) = \frac{1}{2 - \cos x}, \forall x \in R$. Then, find the range of $f$.

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Detailed Solution

Step 1: Analyze the range of the trigonometric component

We know that for any real number $x$, the value of the cosine function lies in the interval $[-1, 1]$. Thus, $-1 \le \cos x \le 1$.

Step 2: Transform the inequality to match the denominator

We need to find the range of $2 - \cos x$. Starting from $-1 \le \cos x \le 1$, we multiply by $-1$ to get $-1 \le -\cos x \le 1$. Adding $2$ to all parts, we get: $$2 - 1 \le 2 - \cos x \le 2 + 1$$ $$1 \le 2 - \cos x \le 3$$

Step 3: Determine the range of the reciprocal function

Since $f(x) = \frac{1}{2 - \cos x}$, we take the reciprocal of the inequality. Taking the reciprocal reverses the inequality signs: $$\frac{1}{3} \le \frac{1}{2 - \cos x} \le \frac{1}{1}$$ Therefore, the range of $f(x)$ is $[\frac{1}{3}, 1]$.

Final Answer: [1/3, 1]

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student must apply the known range of trigonometric functions to a composite algebraic expression.
Knowledge Dimension: PROCEDURAL
Justification: The student follows a step-by-step algorithmic process involving inequality manipulation to derive the range.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. This question tests the conceptual understanding of functions and range, which is a fundamental skill in the Relations and Functions chapter.