CBSE Class 12 Mathematics Relations and Functions Q #2126
KNOWLEDGE BASED
APPLY
3 Marks Generic-Curriculum SA
Prove that the function $f : R \to R$ given by $f(x) = x^3$ is bijective.

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Detailed Solution

Step 1: Understanding Bijectivity

A function is bijective if it is both injective (one-to-one) and surjective (onto).

Step 2: Proving Injective (One-to-One)

Let $x_1, x_2 \in R$ such that $f(x_1) = f(x_2)$. Then:

$$x_1^3 = x_2^3$$

Taking the cube root on both sides, we get $x_1 = x_2$. Since $f(x_1) = f(x_2)$ implies $x_1 = x_2$, the function is injective.

Step 3: Proving Surjective (Onto)

For any $y \in R$ (codomain), we need to find an $x \in R$ (domain) such that $f(x) = y$.

$$x^3 = y \implies x = \sqrt[3]{y}$$

Since the cube root of any real number is a real number, for every $y \in R$, there exists $x = y^{1/3} \in R$ such that $f(x) = (y^{1/3})^3 = y$. Thus, the function is surjective.

Step 4: Conclusion

Since the function is both injective and surjective, it is bijective.

Final Answer: The function is proven to be bijective as it satisfies both one-to-one and onto conditions.

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because it requires the student to apply the formal definitions of injectivity and surjectivity to a specific algebraic function.
Knowledge Dimension: PROCEDURAL
Justification: The student must follow a standard algorithmic proof structure (setting up equations and verifying domain/codomain mapping) to demonstrate the property.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE (Textbook). This is a standard textbook problem found in Chapter 1 (Relations and Functions) designed to test foundational understanding of function mapping.