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Exercise 461Linear algebra › Eigenvalues and diagonalisationHigh schoolAdded 3 Nov 2026

A 2×2 matrix with distinct eigenvalues

#eigenvalues#eigenvectors#distinct roots

Statement

Find the eigenvalues and eigenvectors of
A=(4123).A = \begin{pmatrix} 4 & 1 \\ 2 & 3 \end{pmatrix}.

Hint

Compute the characteristic polynomial det⁡(A−λI)\det(A-\lambda I) and find its roots.

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Solution

Characteristic polynomial.
det⁡(A−λI)=∣4−λ123−λ∣=(4−λ)(3−λ)−2=λ2−7λ+10.\det(A-\lambda I) = \begin{vmatrix} 4-\lambda & 1 \\ 2 & 3-\lambda \end{vmatrix} = (4-\lambda)(3-\lambda)-2 = \lambda^2-7\lambda+10.
The roots are λ1=2\lambda_1=2 and λ2=5\lambda_2=5.

Eigenvector for λ=2\lambda=2.
A−2I=(2121).A-2I = \begin{pmatrix} 2 & 1 \\ 2 & 1 \end{pmatrix}.
The kernel is given by 2x+y=02x+y=0, so v1=(1,−2)v_1=(1,-2).

Eigenvector for λ=5\lambda=5.
A−5I=(−112−2).A-5I = \begin{pmatrix} -1 & 1 \\ 2 & -2 \end{pmatrix}.
The kernel is given by −x+y=0-x+y=0, so v2=(1,1)v_2=(1,1).

Conclusion. Eigenvalues 22 and 55, with eigenvectors (1,−2)(1,-2) and (1,1)(1,1).