1. Determinants and Multiplicative Inverses of Matrices
Section 2-5

2. Before finishing this section you should be able to:
Evaluate determinants
Find inverses of matrices
Solve systems of equations by using inverses of matrices
Remember: Your textbook is your friend! This presentation is just a supplement to the text. BEFORE you view this, make sure you read this section in your textbook and look at all the great examples that are also worked there for you.

3. Determinants
Each square matrix has a determinant. The determinant of
is a number denoted by
or det
The determinant of a 2 x 2 matrix is the difference of the
products of the diagonals. .
ad -
bc

4. = 0(-6) - 8(-2) = 0 + 16 = 16 = (-4)(-8) – (5)(3) = 32 – 15 = 17
Find the value of
= 0(-6) - 8(-2) = = 16
If , find the value of det(Q).
= (-4)(-8) – (5)(3) = 32 – 15 = 17

5. Finding the determinant of a 3x3 matrix
The minor of an element of any nth-order determinant is a determinant of order (n-1). We take the elements in the FIRST ROW and find their minors. The minor can be found by deleting the row and column containing the element.
For instance, find the determinant of
This is called expansion by minors.
Notice that this is SUBTRACTED – the second term is always subtracted
Notice that this is ADDED – the third term is always added

6. 5[4(7)-(-3)(8)] - 3[6(7)-0(8)] + (-1)[6(-3)-0(4)]
Simplify and expand.
5[4(7)-(-3)(8)] - 3[6(7)-0(8)] + (-1)[6(-3)-0(4)]
= 5[28+24] -3[42-0] + (-1)[-18-0]
= 5[52]-3[42]+(-1)[-18] = =152

7. Another Example
Find the value of .
= 2(-8) + 3(-11) – 5(-7)
= -14

8. Identity Matrix
The identity matrix for multiplication is a square matrix whose elements in the main diagonal, from upper left to lower right, are 1’s, and all other elements are 0’s.
Any square matrix A multiplied by the identity matrix, I , will equal A. or
The identity matrix can also be 3 x 3, 4 x 4, 5 x 5, whatever square dimensions you need for that particular problem.

9. Finding the Inverse of a matrix
An inverse matrix, A-1 , is a matrix that, when multiplied by matrix A, produces the identity matrix.
An inverse matrix of a 2 x 2 matrix can be found by multiplying a matrix by the reciprocal of the determinant, switching the 2 numbers of the main diagonal, and changing the signs of the other diagonal, from top right to bottom left.
For example,
Find the inverse of the matrix
An inverse matrix,
First find the determinant  
An inverse exists only if the determinant is not equal to zero !!!!!!!!!!!!
The inverse of the matrix is
The reciprocal of the determinant is multiplied by the altered version of the original matrix. (the elements on the main diagonal switch places, the elements on the other diagonal change signs).

10. Another Example Find the inverse of the matrix.
First, find the determinant of .
= 4(2) - 3(2) or 2
The inverse is

11. Finding the Inverse of a 3 x 3 matrix
We will use our calculator to find this. It is pretty messy by hand.
First, enter your matrix.
Go to 2nd, then MATRIX
Scroll over to EDIT and hit ENTER
Enter the matrix
Hit the x-1 key
To change the decimals to fractions hit MATH then ENTER, then ENTER again
Go to MATRIX, Names and hit ENTER

12. Solving a system of equations using matrices
You can use a matrix inverse to solve a matrix equation in the form AX = B. To solve this equation for X, multiply each side of the equation by the inverse of A.
Solve the system of equations by using matrix equations.
2x + 3y = -17
x – y = 4
Write the system as a matrix equation.

13. coefficient matrix which is
 To solve the matrix equation, first find the inverse of the
coefficient matrix which is
Multiply both sides of the matrix equation by the inverse of the coefficient matrix.

14. Example #2: solving a system of equations using matrices
Solve the system of equations by using matrix equations.
3x + 2y = 3
2x – 4y = 2
Write the system as a matrix equation.
To solve the matrix equation, first find the inverse
of the coefficient matrix.
Now multiply each side of the matrix equation by the inverse and solve.
The solution is (1, 0).

15. Real-world example
BANKING A teller at Security Bank received a deposit from a local retailer containing only twenty-dollar bills and fifty-dollar bills. He received a total of 70 bills, and the amount of the deposit was $3200.
How many bills of each value were deposited?
First, let x represent the number of twenty-dollar bills and let y represent the number of fifty-dollar bills.
So, x + y = 70 since a total of 70 bills were deposited.
Write an equation in standard form that represents the total amount deposited.
20x + 50y = 3,200
Now solve the system of equations x + y = 70 and 20x + 50y = 3200.
Write the system as a matrix equation and solve.

16. Multiply each side of the equation by the
x + y = 70 
20x + 50y = 3200
Multiply each side of the equation by the
inverse of the coefficient matrix.
The solution is (10, 60).
The deposit contained 10 twenty-dollar
bills and 60 fifty-dollar bills.

17. Helpful Websites Finding Determinants/Inverses of Matrices:
Solving systems using matrices:
2-5 self-Check Quiz: