Explain how to find the eigenvalues of the matrix \( \left[\begin{array}{cc} -2 & 1 \\ 18 & 1 \end{array}\right] \).
Answer:
The characteristic polynomial of \( \left[\begin{array}{cc} -2 & 1 \\ 18 & 1 \end{array}\right] \) is \( \lambda^{2} + \lambda - 20 \).
The eigenvalues of \( \left[\begin{array}{cc} -2 & 1 \\ 18 & 1 \end{array}\right] \) are \( -5 \) and \( 4 \).