Class CBSE Class 12 Mathematics Matrices and Determinants Q #1686
COMPETENCY BASED
APPLY
1 Marks 2026 AISSCE(Board Exam) MCQ SINGLE
If A is a non-singular matrix, then which of the following is not true?
(A) adj A is singular
(B) $(adj~A)^{-1}=adj(A^{-1})$
(C) $|A|\ne 0$
(D) $A^{-1}$ exists
Correct Answer: A

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Detailed Solution

Step 1: Analyze the definition of a non-singular matrix

A matrix A is defined as non-singular if its determinant is non-zero, i.e., $|A| \neq 0$. This condition ensures that the inverse matrix A-1 exists. Thus, options (C) and (D) are true statements.

Step 2: Evaluate the property of the adjoint matrix

For any square matrix A, the relationship between the determinant of the adjoint and the original matrix is given by: $$|adj(A)| = |A|^{n-1}$$ where n is the order of the matrix. Since $|A| \neq 0$, it follows that $|adj(A)| \neq 0$. Therefore, adj(A) is also a non-singular matrix. This makes option (A) false.

Step 3: Verify the inverse property

The property $(adj A)^{-1} = adj(A^{-1})$ is a standard identity in matrix algebra for non-singular matrices. Since A is non-singular, A-1 exists, and the identity holds true. Thus, option (B) is true.

Final Answer: (A)

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student must apply properties of determinants and adjoint matrices to evaluate the validity of specific algebraic statements.
Knowledge Dimension: CONCEPTUAL
Justification: The question tests the understanding of the relationship between non-singularity, determinants, and adjoint operations rather than simple rote memorization.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. It requires the student to synthesize multiple properties of matrices (Chapter 4: Determinants) to identify the incorrect statement.