Eigenvectors Matrix
1. **State the problem:** We are given a transformation defined by the matrix $$A = \begin{bmatrix}4 & 2 \\ -1 & 1\end{bmatrix}$$ and need to find its eigenvectors.
2. **Recall the formula:** Eigenvectors $\mathbf{v}$ satisfy $$A\mathbf{v} = \lambda \mathbf{v}$$ where $\lambda$ is an eigenvalue.
3. **Find eigenvalues:** Solve $$\det(A - \lambda I) = 0$$ where $$I = \begin{bmatrix}1 & 0 \\ 0 & 1\end{bmatrix}$$.
Calculate:
$$\det\left(\begin{bmatrix}4-\lambda & 2 \\ -1 & 1-\lambda\end{bmatrix}\right) = (4-\lambda)(1-\lambda) - (-1)(2) = (4-\lambda)(1-\lambda) + 2$$
Expand:
$$(4-\lambda)(1-\lambda) + 2 = 4 - 4\lambda - \lambda + \lambda^2 + 2 = \lambda^2 - 5\lambda + 6$$
Set equal to zero:
$$\lambda^2 - 5\lambda + 6 = 0$$
4. **Solve quadratic:**
$$\lambda = \frac{5 \pm \sqrt{(-5)^2 - 4 \cdot 1 \cdot 6}}{2} = \frac{5 \pm \sqrt{25 - 24}}{2} = \frac{5 \pm 1}{2}$$
So eigenvalues are:
$$\lambda_1 = 3, \quad \lambda_2 = 2$$
5. **Find eigenvectors for each eigenvalue:**
For $\lambda_1 = 3$:
Solve $$ (A - 3I)\mathbf{v} = 0 $$
$$A - 3I = \begin{bmatrix}4-3 & 2 \\ -1 & 1-3\end{bmatrix} = \begin{bmatrix}1 & 2 \\ -1 & -2\end{bmatrix}$$
Set system:
$$1 \cdot x + 2 \cdot y = 0$$
$$-1 \cdot x - 2 \cdot y = 0$$
From first equation:
$$x = -2y$$
Eigenvector corresponding to $\lambda_1=3$ is any scalar multiple of:
$$\mathbf{v}_1 = \begin{bmatrix}-2 \\ 1\end{bmatrix}$$
6. For $\lambda_2 = 2$:
$$A - 2I = \begin{bmatrix}4-2 & 2 \\ -1 & 1-2\end{bmatrix} = \begin{bmatrix}2 & 2 \\ -1 & -1\end{bmatrix}$$
Set system:
$$2x + 2y = 0$$
$$-x - y = 0$$
From first equation:
$$2x = -2y \implies x = -y$$
Eigenvector corresponding to $\lambda_2=2$ is any scalar multiple of:
$$\mathbf{v}_2 = \begin{bmatrix}-1 \\ 1\end{bmatrix}$$
**Final answer:**
Eigenvalues: $3$ and $2$.
Corresponding eigenvectors:
$$\mathbf{v}_1 = \begin{bmatrix}-2 \\ 1\end{bmatrix}, \quad \mathbf{v}_2 = \begin{bmatrix}-1 \\ 1\end{bmatrix}$$