Eigenvalues Eigenvectors 62Ec97
1. **State the problem:** Find the eigenvalues and eigenvectors of the matrix $$A = \begin{bmatrix} 14 & -10 \\ 5 & -1 \end{bmatrix}$$.
2. **Eigenvalues formula:** Eigenvalues $\lambda$ satisfy the characteristic equation $$\det(A - \lambda I) = 0,$$ where $I$ is the identity matrix.
3. **Calculate $A - \lambda I$:**
$$A - \lambda I = \begin{bmatrix} 14 - \lambda & -10 \\ 5 & -1 - \lambda \end{bmatrix}$$
4. **Find the determinant:**
$$\det(A - \lambda I) = (14 - \lambda)(-1 - \lambda) - (-10)(5)$$
$$= (14 - \lambda)(-1 - \lambda) + 50$$
5. **Expand the determinant:**
$$= (14)(-1) + 14(-\lambda) - \lambda(-1) - \lambda^2 + 50$$
$$= -14 - 14\lambda + \lambda - \lambda^2 + 50$$
$$= -\lambda^2 - 13\lambda + 36$$
6. **Set determinant to zero:**
$$-\lambda^2 - 13\lambda + 36 = 0$$
Multiply both sides by $-1$:
$$\lambda^2 + 13\lambda - 36 = 0$$
7. **Solve quadratic equation:**
Using the quadratic formula $$\lambda = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$$ with $a=1$, $b=13$, $c=-36$:
$$\lambda = \frac{-13 \pm \sqrt{13^2 - 4(1)(-36)}}{2} = \frac{-13 \pm \sqrt{169 + 144}}{2} = \frac{-13 \pm \sqrt{313}}{2}$$
8. **Eigenvalues:**
$$\lambda_1 = \frac{-13 + \sqrt{313}}{2}, \quad \lambda_2 = \frac{-13 - \sqrt{313}}{2}$$
9. **Find eigenvectors:** For each eigenvalue $\lambda$, solve $(A - \lambda I)\mathbf{v} = 0$.
10. **Eigenvector for $\lambda_1$:**
$$\begin{bmatrix} 14 - \lambda_1 & -10 \\ 5 & -1 - \lambda_1 \end{bmatrix} \begin{bmatrix} x \\ y \end{bmatrix} = \begin{bmatrix} 0 \\ 0 \end{bmatrix}$$
From the first row:
$$(14 - \lambda_1)x - 10y = 0 \implies y = \frac{14 - \lambda_1}{10} x$$
Choose $x=1$, then
$$y = \frac{14 - \lambda_1}{10}$$
11. **Eigenvector for $\lambda_2$:**
Similarly,
$$(14 - \lambda_2)x - 10y = 0 \implies y = \frac{14 - \lambda_2}{10} x$$
Choose $x=1$, then
$$y = \frac{14 - \lambda_2}{10}$$
**Final answer:**
Eigenvalues:
$$\lambda_1 = \frac{-13 + \sqrt{313}}{2}, \quad \lambda_2 = \frac{-13 - \sqrt{313}}{2}$$
Eigenvectors:
$$\mathbf{v}_1 = \begin{bmatrix} 1 \\ \frac{14 - \lambda_1}{10} \end{bmatrix}, \quad \mathbf{v}_2 = \begin{bmatrix} 1 \\ \frac{14 - \lambda_2}{10} \end{bmatrix}$$