Class CBSE Class 12 Mathematics Matrices and Determinants Q #891
COMPETENCY BASED
APPLY
5 Marks 2023 LA
If $A=\begin{bmatrix}3 & 2\\ 5 & -7\end{bmatrix}$, then find $A^{-1}$ and use it to solve the following system of equations : $3x+5y=11, 2x-7y=-3$.

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Step-by-Step Solution

  1. Find the inverse of matrix A:

    Given $A = \begin{bmatrix} 3 & 2 \\ 5 & -7 \end{bmatrix}$

    The determinant of A, $|A| = (3 \times -7) - (2 \times 5) = -21 - 10 = -31$

    The inverse of A, $A^{-1} = \frac{1}{|A|} \begin{bmatrix} -7 & -2 \\ -5 & 3 \end{bmatrix} = \frac{1}{-31} \begin{bmatrix} -7 & -2 \\ -5 & 3 \end{bmatrix} = \begin{bmatrix} \frac{7}{31} & \frac{2}{31} \\ \frac{5}{31} & \frac{-3}{31} \end{bmatrix}$

  2. Express the system of equations in matrix form:

    The system of equations is: $3x + 5y = 11$ and $2x - 7y = -3$

    In matrix form, this is $AX = B$, where $A = \begin{bmatrix} 3 & 5 \\ 2 & -7 \end{bmatrix}$, $X = \begin{bmatrix} x \\ y \end{bmatrix}$, and $B = \begin{bmatrix} 11 \\ -3 \end{bmatrix}$

  3. Solve for X using $X = A^{-1}B$:

    First, note that the matrix A here is the transpose of the matrix A given in the question. So, we need to find the inverse of this new A.

    Given $A = \begin{bmatrix} 3 & 5 \\ 2 & -7 \end{bmatrix}$

    The determinant of A, $|A| = (3 \times -7) - (5 \times 2) = -21 - 10 = -31$

    The inverse of A, $A^{-1} = \frac{1}{|A|} \begin{bmatrix} -7 & -5 \\ -2 & 3 \end{bmatrix} = \frac{1}{-31} \begin{bmatrix} -7 & -5 \\ -2 & 3 \end{bmatrix} = \begin{bmatrix} \frac{7}{31} & \frac{5}{31} \\ \frac{2}{31} & \frac{-3}{31} \end{bmatrix}$

    Now, $X = A^{-1}B = \begin{bmatrix} \frac{7}{31} & \frac{5}{31} \\ \frac{2}{31} & \frac{-3}{31} \end{bmatrix} \begin{bmatrix} 11 \\ -3 \end{bmatrix} = \begin{bmatrix} \frac{7 \times 11 + 5 \times -3}{31} \\ \frac{2 \times 11 + -3 \times -3}{31} \end{bmatrix} = \begin{bmatrix} \frac{77 - 15}{31} \\ \frac{22 + 9}{31} \end{bmatrix} = \begin{bmatrix} \frac{62}{31} \\ \frac{31}{31} \end{bmatrix} = \begin{bmatrix} 2 \\ 1 \end{bmatrix}$

  4. State the solution:

    Therefore, $x = 2$ and $y = 1$

Correct Answer: x = 2, y = 1

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student needs to apply the concept of matrix inverses to solve a system of linear equations. They must use the formula for finding the inverse and then apply matrix multiplication to find the solution.
Knowledge Dimension: PROCEDURAL
Justification: The question requires the student to follow a specific procedure to find the inverse of a matrix and then use that inverse in a specific procedure to solve a system of equations. This involves knowing the steps and applying them correctly.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. The question requires the application of matrix algebra to solve a real-world problem (system of equations), going beyond simple recall or definition.