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The eigenvalues of the diagonal matrix with entries 2, 3 and 7 are

  • A.42 alone
  • B.0, 0 and 12
  • C.2, 3 and 7
  • D.12 alone

Answer: C

A diagonal matrix is a special case of a triangular one, so its characteristic determinant factorises directly:

\[ \det(A-\lambda I) = (2-\lambda)(3-\lambda)(7-\lambda) = 0 \quad\Rightarrow\quad \lambda = 2,\;3,\;7 \]

Their sum \( 2+3+7=12 \) is the trace and their product \( 2\times3\times7=42 \) the determinant, as the general relations require.

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