Question:medium

Find the eigenvalues of the matrix \(A = \begin{pmatrix} 0 & 0 & -1 \\ 0 & -1 & 0 \\ 0 & 0 & -1 \end{pmatrix}\).

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If a matrix is upper or lower triangular, its eigenvalues are simply the elements on the main diagonal.
While this matrix is not strictly triangular due to the \( -1 \) in the top right, it is effectively block diagonal, making the diagonal elements the roots.
Updated On: Apr 15, 2026
  • \( 0, 1, 1 \)
  • \( 0, -1, -1 \)
  • \( 1, -1, 0 \)
  • \( -1, -1, -1 \)
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The Correct Option is B

Solution and Explanation

To find the eigenvalues of the matrix \(A = \begin{pmatrix} 0 & 0 & -1 \\ 0 & -1 & 0 \\ 0 & 0 & -1 \end{pmatrix}\), we need to solve the characteristic equation, which is given by \(\det(A - \lambda I) = 0\), where \(\lambda\) is the eigenvalue and \(I\) is the identity matrix.

The identity matrix \(I\) of order 3 is:

\(\begin{pmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{pmatrix}\)

Therefore, the matrix \(A - \lambda I\) is:

\(\begin{pmatrix} 0-\lambda & 0 & -1 \\ 0 & -1-\lambda & 0 \\ 0 & 0 & -1-\lambda \end{pmatrix}\)

The determinant of this matrix is:

\(\det(A - \lambda I) = \begin{vmatrix} -\lambda & 0 & -1 \\ 0 & -1-\lambda & 0 \\ 0 & 0 & -1-\lambda \end{vmatrix}\)

Using the properties of determinants, we can calculate it as:

\(\det(A - \lambda I) = (-\lambda) \cdot \left[(-1-\lambda)(-1-\lambda)\right] = (-\lambda)(-1-\lambda)^2\)

Expanding the expression, we have:

\((-\lambda)(1 + 2\lambda + \lambda^2) = -\lambda - 2\lambda^2 - \lambda^3\)

Setting this expression equal to zero for the characteristic equation:

\(-\lambda - 2\lambda^2 - \lambda^3 = 0\)

This can be factored as:

\(\lambda (\lambda^2 + 2\lambda + 1) = 0\)

Factoring further, we get:

\(\lambda (\lambda + 1)^2 = 0\)

The solutions are found by setting each factor to zero:

  • \(\lambda = 0\)
  • \(\lambda + 1 = 0 \Rightarrow \lambda = -1\) (with multiplicity 2)

Thus, the eigenvalues of the matrix are \(0, -1, -1\).

Hence, the correct answer is \(0, -1, -1\).

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