Class CBSE Class 12 Mathematics Relations and Functions Q #571
KNOWLEDGE BASED
APPLY
1 Marks 2024 AISSCE(Board Exam) MCQ SINGLE
A function \(f:\mathbb{R}\rightarrow\mathbb{R}\) defined as \(f(x)=x^{2}-4x+5\) is:
(A) injective but not surjective.
(B) surjective but not injective.
(C) both injective and surjective.
(D) neither injective nor surjective.
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Correct Answer: D

AI Tutor Explanation

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Step-by-Step Solution

Step 1: Analyze the function for injectivity.

A function \(f(x)\) is injective (one-to-one) if for every \(x_1, x_2\) in the domain, \(f(x_1) = f(x_2)\) implies \(x_1 = x_2\). Equivalently, if \(x_1 \neq x_2\), then \(f(x_1) \neq f(x_2)\).

Consider \(f(x) = x^2 - 4x + 5\). We can rewrite this as \(f(x) = (x-2)^2 + 1\). This is a parabola with vertex at \((2, 1)\) opening upwards.

For example, \(f(1) = (1-2)^2 + 1 = 2\) and \(f(3) = (3-2)^2 + 1 = 2\). Since \(f(1) = f(3)\) but \(1 \neq 3\), the function is not injective.

Step 2: Analyze the function for surjectivity.

A function \(f(x)\) is surjective (onto) if for every \(y\) in the codomain, there exists an \(x\) in the domain such that \(f(x) = y\). In other words, the range of the function is equal to the codomain.

Since \(f(x) = (x-2)^2 + 1\), the minimum value of \(f(x)\) is 1 (when \(x = 2\)). Therefore, the range of \(f(x)\) is \([1, \infty)\).

The codomain is \(\mathbb{R}\), which is \((-\infty, \infty)\). Since the range \([1, \infty)\) is not equal to the codomain \((-\infty, \infty)\), the function is not surjective.

Correct Answer: neither injective nor surjective.<\/strong>

AI Suggestion: Option D

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student needs to apply their understanding of injective and surjective functions to the given quadratic function to determine its nature.
Knowledge Dimension: CONCEPTUAL
Justification: The question requires understanding the concepts of injective and surjective functions and how they relate to the properties of a given 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 definitions and properties of injective and surjective functions, which is a core concept covered in the textbook.