Question:medium

If \( \lambda_1, \lambda_2 \) and \( \lambda_3 \) are the eigen values of a square matrix \( A \), then the eigen values of \( A^{-1} + 2I + A \) are

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If \( \lambda \) is an eigenvalue of a matrix \( A \), then any matrix polynomial or functional expression \( f(A) \) will have corresponding eigenvalues given directly by substituting \( \lambda \) into the function, resulting in \( f(\lambda) \).
Updated On: Jul 4, 2026
  • \( \frac{1}{\lambda_1} + 2 + \lambda_1 \), \( \frac{1}{\lambda_2} + 2 + \lambda_2 \) and \( \frac{1}{\lambda_3} + 2 - \lambda_3 \)
  • \( \frac{1}{\lambda_1} + 2 + \lambda_1 \), \( \frac{1}{\lambda_2} - 2 - \lambda_2 \) and \( \frac{1}{\lambda_3} - 2 - \lambda_3 \)
  • \( \frac{1}{\lambda_1} - 2 - \lambda_1 \), \( \frac{1}{\lambda_2} - 2 - \lambda_2 \) and \( \frac{1}{\lambda_3} - 2 - \lambda_3 \)
  • \( \frac{1}{\lambda_1} + 2 + \lambda_1 \), \( \frac{1}{\lambda_2} + 2 + \lambda_2 \) and \( \frac{1}{\lambda_3} + 2 + \lambda_3 \)
Show Solution

The Correct Option is D

Solution and Explanation

Step 1: Think of \( A \) as diagonalizable.
Any square matrix with distinct eigenvalues \( \lambda_1, \lambda_2, \lambda_3 \) can be written as \( A = PDP^{-1} \), where \( D \) is the diagonal matrix of eigenvalues. Once \( A \) is in this form, any polynomial or rational combination of \( A \) is also diagonal in the same basis, so we can work with the diagonal entries directly.

Step 2: Build the diagonal form of the combination.
For \( B = A^{-1} + 2I + A \), writing everything in terms of \( D \) gives \[ B = PD^{-1}P^{-1} + 2PIP^{-1} + PDP^{-1} = P\left(D^{-1} + 2I + D\right)P^{-1} \] So \( B \) has the same eigenvectors as \( A \), and its eigenvalues are simply \( \frac{1}{\lambda_i} + 2 + \lambda_i \) for each \( i \).

Step 3: Check the pattern with a quick numeric example.
Take a simple diagonal matrix with eigenvalues \( 1, 2, 3 \), say \( D = \text{diag}(1,2,3) \). Then \[ D^{-1} + 2I + D = \text{diag}\left(1+2+1,\ \tfrac{1}{2}+2+2,\ \tfrac{1}{3}+2+3\right) = \text{diag}(4,\ 4.5,\ 5.33) \] Each entry is exactly \( \frac{1}{\lambda} + 2 + \lambda \) for that eigenvalue, with no sign change anywhere.

So the eigenvalues of \( A^{-1} + 2I + A \) are \[ \boxed{\frac{1}{\lambda_1} + 2 + \lambda_1,\ \ \frac{1}{\lambda_2} + 2 + \lambda_2,\ \ \frac{1}{\lambda_3} + 2 + \lambda_3} \]
which is option (D).
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