## Find the Inverse of a Matrix

### Learning Outcomes

• Verify that multiplying a matrix by its inverse results in 1.
• Use matrix multiplication to find the inverse of a matrix.
• Find an inverse by augmenting with an identity matrix.

We know that the multiplicative inverse of a real number $a$ is ${a}^{-1}$ and $a{a}^{-1}={a}^{-1}a=\left(\frac{1}{a}\right)a=1$. For example, ${2}^{-1}=\frac{1}{2}$ and $\left(\frac{1}{2}\right)2=1$. The multiplicative inverse of a matrix is similar in concept, except that the product of matrix $A$ and its inverse ${A}^{-1}$ equals the identity matrix. The identity matrix is a square matrix containing ones down the main diagonal and zeros everywhere else. We identify identity matrices by ${I}_{n}$ where $n$ represents the dimension of the matrix. The equations below are the identity matrices for a $2\text{}\times \text{}2$ matrix and $3\text{}\times \text{}3$ matrix, respectively.

${I}_{2}=\left[\begin{array}{rrr}\hfill 1& \hfill & \hfill 0\\ \hfill 0& \hfill & \hfill 1\end{array}\right]$

${I}_{3}=\left[\begin{array}{rrrrr}\hfill 1& \hfill & \hfill 0& \hfill & \hfill 0\\ \hfill 0& \hfill & \hfill 1& \hfill & \hfill 0\\ \hfill 0& \hfill & \hfill 0& \hfill & \hfill 1\end{array}\right]$

The identity matrix acts as a 1 in matrix algebra. For example, $AI=IA=A$.

A matrix that has a multiplicative inverse has the properties

$\begin{array}{l}A{A}^{-1}=I\\ {A}^{-1}A=I\end{array}$

A matrix that has a multiplicative inverse is called an invertible matrix. Only a square matrix may have a multiplicative inverse, as the reversibility, $A{A}^{-1}={A}^{-1}A=I$, is a requirement. Not all square matrices have an inverse, but if $A$ is invertible, then ${A}^{-1}$ is unique. We will look at two methods for finding the inverse of a $2\text{}\times \text{}2$ matrix and a third method that can be used on both $2\text{}\times \text{}2$ and $3\text{}\times \text{}3$ matrices.

### A General Note: The Identity Matrix and Multiplicative Inverse

The identity matrix, ${I}_{n}$, is a square matrix containing ones down the main diagonal and zeros everywhere else.

$\begin{array}{l}\begin{array}{l}\hfill \\ {I}_{2} & =\left[\begin{array}{rr}\hfill 1& \hfill 0\\ \hfill 0& \hfill 1\end{array}\right]\begin{array}{cccc}& & & \end{array} & {I}_{3} & =\left[\begin{array}{rrr}\hfill 1& \hfill 0& \hfill 0\\ \hfill 0& \hfill 1& \hfill 0\\ \hfill 0& \hfill 0& \hfill 1\end{array}\right]\hfill \\ & \text{}2\times 2 & & \text{3}\times 3\hfill \end{array}\hfill \end{array}$

If $A$ is an $n\times n$ matrix and $B$ is an $n\times n$ matrix such that $AB=BA={I}_{n}$, then $B={A}^{-1}$, the multiplicative inverse of a matrix $A$.

### tip for success

Work through each of the examples below on paper, then apply the information in the General Note and How To boxes.

Repeating this process a few times will help you to gain familiarity with the material.

Remember, practice and patient effort enable understanding. Don’t be discouraged if you don’t “get it” right away!

### Example: Showing That the Identity Matrix Acts as a 1

Given matrix A, show that $AI=IA=A$.
$A=\left[\begin{array}{cc}3& 4\\ -2& 5\end{array}\right]$

### How To: Given two matrices, show that one is the multiplicative inverse of the other

1. Given matrix $A$ of order $n\times n$ and matrix $B$ of order $n\times n$ multiply $AB$.
2. If $AB=I$, then find the product $BA$. If $BA=I$, then $B={A}^{-1}$ and $A={B}^{-1}$.

### Example: Showing That Matrix A Is the Multiplicative Inverse of Matrix B

Show that the given matrices are multiplicative inverses of each other.

$A=\left[\begin{array}{rrr}\hfill 1& \hfill & \hfill 5\\ \hfill -2& \hfill & \hfill -9\end{array}\right],B=\left[\begin{array}{rrr}\hfill -9& \hfill & \hfill -5\\ \hfill 2& \hfill & \hfill 1\end{array}\right]$

### Try It

Show that the following two matrices are inverses of each other.

$A=\left[\begin{array}{rrr}\hfill 1& \hfill & \hfill 4\\ \hfill -1& \hfill & \hfill -3\end{array}\right],B=\left[\begin{array}{rrr}\hfill -3& \hfill & \hfill -4\\ \hfill 1& \hfill & \hfill 1\end{array}\right]$

### tip for success

The following sections offer examples of various efficient techniques that show the power of matrix operations. Again, work through them slowly and carefully as needed to gain familiarity.

These techniques, using matrix multiplication, augmenting a matrix with the identity then reducing, and using a formula to find the inverse matrix appear frequently in scientific application and are worth taking time to obtain.

## Finding the Multiplicative Inverse Using Matrix Multiplication

We can now determine whether two matrices are inverses, but how would we find the inverse of a given matrix? Since we know that the product of a matrix and its inverse is the identity matrix, we can find the inverse of a matrix by setting up an equation using matrix multiplication.

### Example: Finding the Multiplicative Inverse Using Matrix Multiplication

Use matrix multiplication to find the inverse of the given matrix.

$A=\left[\begin{array}{rrr}\hfill 1& \hfill & \hfill -2\\ \hfill 2& \hfill & \hfill -3\end{array}\right]$

## Finding the Multiplicative Inverse by Augmenting with the Identity

Another way to find the multiplicative inverse is by augmenting with the identity. When matrix $A$ is transformed into $I$, the augmented matrix $I$ transforms into ${A}^{-1}$.

For example, given

$A=\left[\begin{array}{rrr}\hfill 2& \hfill & \hfill 1\\ \hfill 5& \hfill & \hfill 3\end{array}\right]$

augment $A$ with the identity

$\left[\begin{array}{cc|cc}\hfill 2& \hfill 1& \hfill 1& \hfill 0\\ \hfill 5& \hfill 3& \hfill 0& \hfill 1\\ \end{array}\right]$

Perform row operations with the goal of turning $A$ into the identity.

1. Switch row 1 and row 2.
$\left[\begin{array}{cc|cc}\hfill 5& \hfill 3& \hfill 0& \hfill 1\\ \hfill 2& \hfill 1& \hfill 1& \hfill 0\\ \end{array}\right]$
2. Multiply row 2 by $-2$ and add to row 1.
$\left[\begin{array}{cc|cc}\hfill 1& \hfill 1& \hfill -2& \hfill 1\\ \hfill 2& \hfill 1& \hfill 1& \hfill 0\\ \end{array}\right]$
3. Multiply row 1 by $-2$ and add to row 2.
$\left[\begin{array}{cc|cc}\hfill 1& \hfill 1& \hfill -2& \hfill 1\\ \hfill 0& \hfill -1& \hfill 5& \hfill -2\\ \end{array}\right]$
4. Add row 2 to row 1.
$\left[\begin{array}{cc|cc}\hfill 1& \hfill 0& \hfill 3& \hfill -1\\ \hfill 0& \hfill -1& \hfill 5& \hfill -2\\ \end{array}\right]$
5. Multiply row 2 by $-1$.
$\left[\begin{array}{cc|cc}\hfill 1& \hfill 0& \hfill 3& \hfill -1\\ \hfill 0& \hfill 1& \hfill -5& \hfill 2\\ \end{array}\right]$

The matrix we have found is ${A}^{-1}$.

${A}^{-1}=\left[\begin{array}{rrr}\hfill 3& \hfill & \hfill -1\\ \hfill -5& \hfill & \hfill 2\end{array}\right]$

## Finding the Multiplicative Inverse of 2×2 Matrices Using a Formula

When we need to find the multiplicative inverse of a $2\times 2$ matrix, we can use a special formula instead of using matrix multiplication or augmenting with the identity.

If $A$ is a $2\times 2$ matrix, such as

$A=\left[\begin{array}{rrr}\hfill a& \hfill & \hfill b\\ \hfill c& \hfill & \hfill d\end{array}\right]$

the multiplicative inverse of $A$ is given by the formula

${A}^{-1}=\frac{1}{ad-bc}\left[\begin{array}{rrr}\hfill d& \hfill & \hfill -b\\ \hfill -c& \hfill & \hfill a\end{array}\right]$

where $ad-bc\ne 0$. If $ad-bc=0$, then $A$ has no inverse.

### Example: Using the Formula to Find the Multiplicative Inverse of Matrix A

Use the formula to find the multiplicative inverse of

$A=\left[\begin{array}{cc}1& -2\\ 2& -3\end{array}\right]$

### Try It

Use the formula to find the inverse of matrix $A$. Verify your answer by augmenting with the identity matrix.

$A=\left[\begin{array}{cc} \hfill 1& \hfill -1\\ \hfill 2& \hfill 3\end{array}\right]$

### Example: Finding the Inverse of the Matrix, If It Exists

Find the inverse, if it exists, of the given matrix.

$A=\left[\begin{array}{cc}3& 6\\ 1& 2\end{array}\right]$

### Finding the Multiplicative Inverse of 3×3 Matrices

Unfortunately, we do not have a formula similar to the one for a $2\text{}\times \text{}2$ matrix to find the inverse of a $3\text{}\times \text{}3$ matrix. Instead, we will augment the original matrix with the identity matrix and use row operations to obtain the inverse.

Given a $3\text{}\times \text{}3$ matrix

$A=\left[\begin{array}{ccc}2& 3& 1\\ 3& 3& 1\\ 2& 4& 1\end{array}\right]$

augment $A$ with the identity matrix

$A|I=\left[\begin{array}{ccc|ccc}\hfill 2& \hfill 3& \hfill 1& \hfill 1& \hfill 0& \hfill 0\\ \hfill 3& \hfill 3& \hfill 1& \hfill 0& \hfill 1& \hfill 0\\ \hfill 2& \hfill 4& \hfill 1& \hfill 0& \hfill 0& \hfill 1\end{array}\right]$

To begin, we write the augmented matrix with the identity on the right and $A$ on the left. Performing elementary row operations so that the identity matrix appears on the left, we will obtain the inverse matrix on the right. We will find the inverse of this matrix in the next example.

### How To: Given a $3\times 3$ matrix, find the inverse

1. Write the original matrix augmented with the identity matrix on the right.
2. Use elementary row operations so that the identity appears on the left.
3. What is obtained on the right is the inverse of the original matrix.
4. Use matrix multiplication to show that $A{A}^{-1}=I$ and ${A}^{-1}A=I$.

### Example: Finding the Inverse of a 3 × 3 Matrix

Given the $3\times 3$ matrix $A$, find the inverse.

$A=\left[\begin{array}{ccc}2& 3& 1\\ 3& 3& 1\\ 2& 4& 1\end{array}\right]$

### Try It

Find the inverse of the $3\times 3$ matrix.

$A=\left[\begin{array}{ccc}\hfill 2&\hfill -17& \hfill 11\\ \hfill -1& \hfill 11& \hfill -7\\ \hfill 0& \hfill 3& \hfill -2\end{array}\right]$