Question
Find eigenvectors of matrix:
$$ A = \left[ \begin{matrix}8&-6&2\\-6&7&-4\\2&-4&3\end{matrix} \right] $$Answer
The eigenvectors and eigenvalues of matrix A are:
| Eigenvalue | Eigenvector |
| $ 15 $ | $ \begin{bmatrix} 1 \cr -1 \cr {{1}\over{2}} \end{bmatrix} $ |
| $ 3 $ | $ \begin{bmatrix} 1 \cr {{1}\over{2}} \cr -1 \end{bmatrix} $ |
| $ 0 $ | $ \begin{bmatrix} 1 \cr 2 \cr 2 \end{bmatrix} $ |
Explanation
Step 1 : Find characteristic polynomial $ p(\lambda) $:
$$ p(\lambda) = -\lambda^3+18\lambda^2-45\lambda $$Step 2 : Find the eigenvalues by solving the characteristic equation $ -\lambda^3+18\lambda^2-45\lambda = 0. $
The eigenvalues are
$$ \lambda_1 = 15 ~ , ~ \lambda_2 = 3 ~ , ~ \lambda_3 = 0 $$( click here to view an explanation on how to solve this equation.)
Step 3 : To find the associated eigenvectors ( $ x $ ), we have to solve the equation $ A x = \lambda I $.
For $ \lambda_ 0 = 15 $ we have $ A x = 15 I $.
For $ \lambda_ 1 = 3 $ we have $ A x = 3 I $.
For $ \lambda_ 2 = 0 $ we have $ A x = 0 I $.
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