In the previous article, you learned about inverse of matrix and why it is important. You will learn how to find inverse of a matrix in this article. There are two primary method of finding inverse of any square invertible matrix – Classical adjoint method and Gauss -Jordan elimination method.
Methods To Find Inverse Of Matrix
The primary method of finding of inverse of matrix are
- Classical adjoint method
- Gauss-Jordan elimination method
Throughout this article, we will discuss about these two methods in detail.
Classical Adjoint Method
If is an invertible matrix then we can find the inverse of matrix with the adjoint of matrix .
But,before we begin you must understand a few terminologies.
Determinant – It is a special number obtained from a square matrix, non-square matrix do not have determinants. If is a square matrix then there is a number of ways to denote its determinant.
Minor of a matrix – The minor of a square matrix is determinant obtained by deleting a row and a column from the determinant of a larger square matrix. It is denoted by for element where is the ithrow and is the jth column.
The determinant of 2 x 2 matrix You get following matrix of determinants
Cofactor of a Matrix – The cofactor of a matrix can be found from determinant of minors by assigning appropriate negative or positive signs. Each cofactor can be found by following equation.
The matrix of cofactors of A
Adjoint of a Matrix – The adjoint of a matrix is obtained by transposing the co-factor matrix. It is denoted as .
Finding Inverse Of Matrix Through Adjoint Method
The process of finding inverse of matrix using adjoint method is as follows.
- Find the matrix of minors
- Find the matrix of co-factors
- Find the determinant det(A) by multiplying first row of matrix A with first row of co-factor matrix of A.
- Find the adjoint matrix usign co-factor matrix of A.
- Multiply 1/det(A) with adjoint of A (adj A) to get the inverse matrix of A.
We will find inverse of a matrix using the adjoint of matrix in the next section. First we must find the inverse of matrix, then and finally matrix.
Example #1 : Find the inverse of matrix using adjoint method.
Let be a matrix of order 1 x 1. Step 1: There is no minor for 1x1 matrix. Step 2: There is no cofactor for 1x1 matrix. Step 3: The determinant of A is .(invertible) Step 4: The adjoint of matrix A is . To compute the inverse of matrix A use
Example #2 : Find the inverse of matrix using adjoint method.
Let be a square and invertible matrix of order .
Step 1: The minors of matrix are
The minor matrix is Step 2: The cofactor of matrix is obtained from minor matrix. The cofactor matrix is Step 3: To compute the determinant simply multiply corresponding elements of top row of matrix and tow row of cofactor of and add them. Top row of matrix B = 2, 1 Top row of cofactor matrix = 7 -3 Step 4: Find the adjoint of the matrix by transposing cofactor matrix. To find the inverse of matrix use following Verify the inverse of matrix
Example #3 : Find the inverse of 3 x 3 matrix using the adjoint method.
Given the matrix of order .
Step 1: Find the minors of the matrix .
We have the following matrix of minors
Step 2: Find the cofactors of matrix .
We can use the matrix of minors to find the matrix of cofactors. We get the cofactor matrix of
Step 3: Find determinant of the matrix .
Top row of matrix A = 1, -2, 1 Top row of coactor of A = 5, -18, 2
Step 4: Find the adjoint of matrix . The adjoint of the matrix can be obtained from transposing the cofactor matrix.
Step 5: Find the inverse of matrix using following equation.
Finding Inverse of Matrix Using Gauss-Jordan Elimination Method
The Gauss-Jordan elimination method convert a matrix into reduced-row echelon form to find the value of solution vector in , but here we use the technique to find inverse matrix .
The Gauss-Jordan technique involves row operations on augmented matrix obtained from the system of linear equations that transforms the matrix into identity matrix where is the order of matrix .
These row operations are
- Multiply a row with scalar where .
- Interchange row with row .
- Addition of times row to row .
Example #5 : Row operations
Let be matrix of order . Multiply a row 1 with 3. Interchange row 2 with row 1. Add 2 times row 1 to row 2.
An elementary matrix is a matrix obtained from performing a single row operation on identity matrix .
Suppose A is a matrix of order , then the product is same as performing that row operation on matrix .
Example #6 : Elementary Matrix
Suppose is a matrix of order . Let be row operation on of order . Let Let us perform on . (1) Now, let us find matrix. (2) Therefore, (1) and (2) are same.
If matrix is a result of row operation on identity matrix , then there exist some operation if performed on will give back . Such an operation is called an Inverse operation.
For every elementary row operation, there is an equivalent inverse operation. Check the table below.
|Elementary Row Operation||Inverse Operation|
|Multiply row i by c where c != 0||Multiply row i by 1/c|
|Interchange row i with row j||Interchange row j with row i|
|Add k times row i to row j||Add -k times row i to row j|
Example #7 : Inverse Operations
Multiply row by . Multiply row by where
From the above we can conclude that
1. Some operation on give the elementary matrix . 2. The inverse of previous operation on E will give back . 3. Inverse operation on will result in an inverse matrix such that Note that is invertible matrix and has determinant greater than 0.
4 Necessary Statements For Gauss-Jordan technique and inverses
Before we find inverse of a matrix using Gauss-Jordan technique, there are 4 necessary prepositions that must be true about the matrix and its inverse.
If matrix is square matrix these 4 statement must always be true.
- Matrix is invertible meaning the determinant is greater than 0 and does not have a zero row.
- has only trivial solution.
- The matrix is reduced to reduced-row echelon form which is identity matrix . This is because has trivial solution only, which means has one solution for every .
- The matrix is product of elementary matrices.
Multiplying A with elementary matrices is same as performing row operations on A which will reduce the matrix to identity matrix . Similarly, each of the is invertible, a series of inverse operation on I will give back . also,
Therefore, the matrix A$ is product of elementary matrices.
Example #8: Find the inverse of following matrix using Gauss-Jordan elimination method.
Given the matrix we will find the inverse of matrix using the Gauss-Jordan elimination method.
Step 1: Step 2: Step 3: Step 4: Step 5: Step 6: Step 7: Step 8:
The matrix is reduced to an identity matrix and the identity matrix after Gauss-Jordan elimination method reduced to inverse matrix of .
The verification of resultant inverse matrix is left as an exercise for you.