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The eigenvalues of the matrix \(\begin{bmatrix} 1 & 2 \\ 0 & 3 \end{bmatrix}\) are _____.

  • A.0, 2
  • B.2, 3
  • C.1, 3
  • D.1, 2

Answer: C

The eigenvalues are 1 and 3.

The matrix is upper triangular, and the eigenvalues of a triangular matrix are its diagonal entries. Formally:

\[ \det\begin{bmatrix} 1 - \lambda & 2 \\ 0 & 3 - \lambda \end{bmatrix} = (1 - \lambda)(3 - \lambda) = 0 \]

Check: the eigenvalues sum to the trace, 1 + 3 = 4, and multiply to the determinant, 3. The pairs (0, 2), (2, 3) and (1, 2) fail one or both of these checks; the off-diagonal 2 does not affect the eigenvalues of a triangular matrix.

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