Class CBSE Class 12 Mathematics Continuity and Differentiability Q #974
KNOWLEDGE BASED
REMEMBER
1 Marks 2025 AISSCE(Board Exam) ASSERTION REASON
Assertion: Assertion (A): $f(x) = \begin{cases} x\sin\frac{1}{x}, & x\neq 0 \\ 0, & x=0 \end{cases}$ is continuous at $x=0$.
Reason: Reason (R): When $x \to 0$, $\sin\frac{1}{x}$ is a finite value between $-1$ and $1$.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false but R is true.

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Detailed Solution

Step 1: Analyze Assertion (A)

To check the continuity of $f(x)$ at $x=0$, we need to verify if $\lim_{x \to 0} f(x) = f(0)$. Since $f(0) = 0$, we need to find $\lim_{x \to 0} x\sin\frac{1}{x}$.

Step 2: Evaluate the limit

We know that $-1 \leq \sin\frac{1}{x} \leq 1$ for all $x \neq 0$. Therefore, $-|x| \leq x\sin\frac{1}{x} \leq |x|$. As $x \to 0$, we have $-|x| \to 0$ and $|x| \to 0$. By the Squeeze Theorem, $\lim_{x \to 0} x\sin\frac{1}{x} = 0$. Since $\lim_{x \to 0} f(x) = 0 = f(0)$, the function $f(x)$ is continuous at $x=0$. Thus, Assertion (A) is true.

Step 3: Analyze Reason (R)

Reason (R) states that when $x \to 0$, $\sin\frac{1}{x}$ is a finite value between $-1$ and $1$. This statement is true because the sine function always oscillates between -1 and 1, regardless of how small $x$ becomes. However, the limit $\lim_{x \to 0} \sin\frac{1}{x}$ does not exist, but $\sin\frac{1}{x}$ remains bounded between -1 and 1.

Step 4: Determine the relationship between Assertion and Reason

The assertion is true, and the reason is also true. However, the reason correctly explains why the limit of $x\sin\frac{1}{x}$ as $x$ approaches 0 exists and is equal to 0, which makes the function continuous. Therefore, the reason is the correct explanation for the assertion.

Final Answer: Both A and R are true and R is the correct explanation of A.

AI Suggestion: Option A

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Pedagogical Audit
Bloom's Analysis: This is an REMEMBER question because it requires recalling the definition of continuity and the properties of the sine function.
Knowledge Dimension: CONCEPTUAL
Justification: The question tests the understanding of the concept of continuity and the properties of trigonometric functions, specifically the sine function. It requires applying the definition of continuity using limits.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE. It directly relates to the chapter on continuity and differentiability, focusing on the definition and conditions for continuity of a function at a point.
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