Question:medium

If $\begin{bmatrix} 2 & 3 \\ 5 & 7 \end{bmatrix} \begin{bmatrix} 1 & -3 \\ -2 & 4 \end{bmatrix} = \begin{bmatrix} -4 & 6 \\ -9 & x \end{bmatrix}$, then the value of $x$ is:

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Matrix multiplication is row-by-column; verify each element carefully.
Updated On: Apr 2, 2026
  • 13
  • -13
  • 9
  • 5
Show Solution

The Correct Option is B

Solution and Explanation

Perform matrix multiplication: \[ \begin{bmatrix} 2 & 3 \\ 5 & 7 \end{bmatrix} \begin{bmatrix} 1 & -3 \\ -2 & 4 \end{bmatrix} = \begin{bmatrix} 2 \times 1 + 3 \times (-2) & 2 \times (-3) + 3 \times 4 \\ 5 \times 1 + 7 \times (-2) & 5 \times (-3) + 7 \times 4 \end{bmatrix} \] Calculate each element of the resulting matrix: \[ a_{11} = 2 - 6 = -4 \] \[ a_{12} = -6 + 12 = 6 \] \[ a_{21} = 5 - 14 = -9 \] \[ a_{22} = -15 + 28 = 13 \] The resulting matrix is: \[ \begin{bmatrix} -4 & 6 \\ -9 & x \end{bmatrix} \] By equating the calculated elements with the unknown matrix, we find: \[ x = 13 \] This result, \(x = 13\), corresponds to option (A). However, the provided options also include -13. A review of the calculation for \(a_{22}\) confirms: \[ a_{22} = 5 \times (-3) + 7 \times 4 = -15 + 28 = 13 \] Therefore, the value of \(x\) is definitively 13.
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