Class CBSE Class 12 Mathematics Relations and Functions Q #573
KNOWLEDGE BASED
UNDERSTAND
1 Marks 2024 AISSCE(Board Exam) MCQ SINGLE
Let \(f:R_{+}\rightarrow[-5,\infty)\) be defined as \(f(x)=9x^{2}+6x-5\), where \(R_{+}\) is the set of all non-negative real numbers. Then, f is:
(A) one-one
(B) onto
(C) bijective
(D) neither one-one nor onto
Correct Answer: C

AI Tutor Explanation

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Detailed Solution

Step 1: Analyze the function

The given function is \(f(x) = 9x^2 + 6x - 5\), defined for \(x \in R_{+}\), where \(R_{+}\) represents non-negative real numbers, and the codomain is \([-5, \infty)\).

Step 2: Check for one-one (injectivity)

To check if the function is one-one, we can analyze its derivative. The derivative of \(f(x)\) is \(f'(x) = 18x + 6\). Since \(x \in R_{+}\), \(x \geq 0\), so \(18x + 6 \geq 6 > 0\). This means that \(f'(x) > 0\) for all \(x \in R_{+}\). Therefore, the function is strictly increasing on \(R_{+}\). A strictly increasing function is always one-one (injective).

Step 3: Check for onto (surjectivity)

To check if the function is onto, we need to determine if the range of the function is equal to the codomain. The codomain is given as \([-5, \infty)\). Let's find the minimum value of the function. Since \(f(x) = 9x^2 + 6x - 5\), we can complete the square: \(f(x) = (3x)^2 + 2(3x)(1) + 1 - 1 - 5 = (3x + 1)^2 - 6\) Since \(x \in R_{+}\), the minimum value of \(3x + 1\) is 1 (when \(x = 0\)). Therefore, the minimum value of \((3x + 1)^2\) is \(1^2 = 1\). So, the minimum value of \(f(x)\) is \(1 - 6 = -5\). As \(x\) increases, \(f(x)\) also increases. Thus, the range of \(f(x)\) is \([-5, \infty)\), which is equal to the codomain. Therefore, the function is onto (surjective).

Step 4: Check for bijective

Since the function is both one-one (injective) and onto (surjective), it is bijective.

Final Answer: bijective

AI Suggestion: Option C

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Pedagogical Audit
Bloom's Analysis: This is an UNDERSTAND question because the student needs to understand the concepts of one-one, onto, and bijective functions and apply calculus to determine the function's behavior.
Knowledge Dimension: CONCEPTUAL
Justification: The question requires understanding the concepts of functions, injectivity, surjectivity, and bijectivity, as well as the application of calculus to analyze the function's behavior.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE. The question directly tests the student's understanding of relations and functions, specifically the concepts of one-one, onto, and bijective functions.
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