GRE Subject Test: Math : Eigenvalues

Study concepts, example questions & explanations for GRE Subject Test: Math

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Eigenvalues

Find the eigenvalues of the following matrix, if possible.  

\(\displaystyle A=\begin{bmatrix} 1&2 \\ -2&1 \end{bmatrix}\)

Possible Answers:

\(\displaystyle \lambda=\frac{5}{2}\)

\(\displaystyle \lambda_{1,2}=\pm1\)

\(\displaystyle \lambda_{1,2}=1\pm2i\)

The eigenvalues do not exist.

\(\displaystyle \lambda=\begin{bmatrix} 2i&4i \\ -2i&i \end{bmatrix}\)

Correct answer:

\(\displaystyle \lambda_{1,2}=1\pm2i\)

Explanation:

In order to find the eigenvalues of a matrix, apply the following formula:

\(\displaystyle det(A-\lambda I) =0\)

\(\displaystyle I\) is the identity matrix.

\(\displaystyle \begin{bmatrix} 1&2 \\ -2&1 \end{bmatrix}-\lambda \begin{bmatrix} 1&0 \\ 0&1 \end{bmatrix}=\begin{bmatrix} 1-\lambda & 2\\ -2&1-\lambda \end{bmatrix}\)

Compute the determinant and set it equal to zero.

\(\displaystyle (1-\lambda)(1-\lambda)-(2)(-2)=0\)

\(\displaystyle 1-2\lambda+\lambda^2+4=0\)

\(\displaystyle \lambda^2-2\lambda+5=0\)

Solve for lambda by using the quadratic formula.

\(\displaystyle \lambda_{1,2}=1\pm2i\)

 

Example Question #2 : Eigenvalues

Find the eigenvalues of the following matrix, if possible.  

\(\displaystyle A=\begin{bmatrix} 1&2 \\ -2&1 \end{bmatrix}\)

Possible Answers:

\(\displaystyle \lambda=\begin{bmatrix} 2i&4i \\ -2i&i \end{bmatrix}\)

\(\displaystyle \lambda=\frac{5}{2}\)

\(\displaystyle \lambda_{1,2}=\pm1\)

The eigenvalues do not exist.

\(\displaystyle \lambda_{1,2}=1\pm2i\)

Correct answer:

\(\displaystyle \lambda_{1,2}=1\pm2i\)

Explanation:

In order to find the eigenvalues of a matrix, apply the following formula:

\(\displaystyle det(A-\lambda I) =0\)

\(\displaystyle I\) is the identity matrix.

\(\displaystyle \begin{bmatrix} 1&2 \\ -2&1 \end{bmatrix}-\lambda \begin{bmatrix} 1&0 \\ 0&1 \end{bmatrix}=\begin{bmatrix} 1-\lambda & 2\\ -2&1-\lambda \end{bmatrix}\)

Compute the determinant and set it equal to zero.

\(\displaystyle (1-\lambda)(1-\lambda)-(2)(-2)=0\)

\(\displaystyle 1-2\lambda+\lambda^2+4=0\)

\(\displaystyle \lambda^2-2\lambda+5=0\)

Solve for lambda by using the quadratic formula.

\(\displaystyle \lambda_{1,2}=1\pm2i\)

Learning Tools by Varsity Tutors