Class CBSE Class 12 Mathematics Matrices and Determinants Q #733
KNOWLEDGE BASED
UNDERSTAND
1 Marks 2024 MCQ SINGLE
Given that \(A^{-1}=\frac{1}{7}\begin{bmatrix}2&1\\ -3&2\end{bmatrix}\) matrix A is
(A) \(7[\begin{matrix}2&-1\\ 3&2\end{matrix}]\)
(B) \([\begin{matrix}2&-1\\ 3&2\end{matrix}]\)
(C) \(\frac{1}{7}[\begin{matrix}2&-1\\ 3&2\end{matrix}]\)
(D) \(\frac{1}{49}[\begin{matrix}2&-1\\ 3&2\end{matrix}]\)

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Step-by-Step Solution

Given: \(A^{-1} = \frac{1}{7}\begin{bmatrix} 2 & 1 \\ -3 & 2 \end{bmatrix}\)

To find matrix A, we need to find the inverse of \(A^{-1}\).

Let \(B = A^{-1} = \frac{1}{7}\begin{bmatrix} 2 & 1 \\ -3 & 2 \end{bmatrix}\)

First, find the determinant of the matrix inside the scalar multiplication:

\(\text{det}\begin{bmatrix} 2 & 1 \\ -3 & 2 \end{bmatrix} = (2 \times 2) - (1 \times -3) = 4 + 3 = 7\)

Now, find the inverse of the matrix inside the scalar multiplication:

\(\begin{bmatrix} 2 & 1 \\ -3 & 2 \end{bmatrix}^{-1} = \frac{1}{7}\begin{bmatrix} 2 & -1 \\ 3 & 2 \end{bmatrix}\)

Since \(B = \frac{1}{7}\begin{bmatrix} 2 & 1 \\ -3 & 2 \end{bmatrix}\), then \(B^{-1} = \left(\frac{1}{7}\begin{bmatrix} 2 & 1 \\ -3 & 2 \end{bmatrix}\right)^{-1} = 7 \begin{bmatrix} 2 & 1 \\ -3 & 2 \end{bmatrix}^{-1} = 7 \cdot \frac{1}{7} \begin{bmatrix} 2 & -1 \\ 3 & 2 \end{bmatrix} = \begin{bmatrix} 2 & -1 \\ 3 & 2 \end{bmatrix}\)

Therefore, \(A = \begin{bmatrix} 2 & -1 \\ 3 & 2 \end{bmatrix}\)

Correct Answer: \(\begin{bmatrix}2&-1\\ 3&2\end{bmatrix}\)

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Pedagogical Audit
Bloom's Analysis: This is an UNDERSTAND question because the student needs to understand the relationship between a matrix and its inverse to find the original matrix A. They need to apply the concept of finding the inverse of a matrix.
Knowledge Dimension: PROCEDURAL
Justification: The question requires the student to apply a specific procedure (finding the inverse of a matrix) to solve the problem. It involves knowing the steps and algorithms to manipulate matrices.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE. The question directly tests the student's understanding of matrix inverses, a concept covered in the textbook.

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