Question:

If matrix $A = \begin{bmatrix} 1 & -1 \\ -1 & 1 \end{bmatrix}$ and $A^2 = kA$, then the value of $k$ is:

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Always multiply carefully and match the matrix form.
  • 1
  • -2
  • 2
  • -1
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The Correct Option is C

Solution and Explanation

Compute $A^2$: \[ A^2 = A \cdot A = \begin{bmatrix} 1 & -1 \\ -1 & 1 \end{bmatrix} \begin{bmatrix} 1 & -1 \\ -1 & 1 \end{bmatrix} = \begin{bmatrix} (1)(1)+(-1)(-1) & (1)(-1)+(-1)(1) \\ (-1)(1)+(1)(-1) & (-1)(-1)+(1)(1) \end{bmatrix} = \begin{bmatrix} 2 & -2 \\ -2 & 2 \end{bmatrix}. \] So, \[ A^2 = 2A \implies k = 2. \]
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