warm-up
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Warm-Up
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3541
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3) Find the determinant by hand.
14541310250
4) Find the determinant using your calculator.
1) Multiply. Show work.
2) Multiply. Show work.
Daily Quiz – you may not use your notes.
122433692
2 3 4
3) Find the determinant by hand.
4) Find the determinant using your calculator.
1) Multiply. Show work.
2) Multiply. Show work.
85
212
1428601010415
1201
102333
4.4 Identity & Inverse 4.4 Identity & Inverse MatricesMatrices
InversesWhat does “inverse” mean to you?What is the inverse of multiplication?
Identity
In other words, 5 * __= 5?
What does “identity” mean to you?What is the multiplicative identity for the real numbers?
The identity for multiplication is 1 because anything multiplied by 1 will be itself.
a * a-1= 1
In other words, 5 * ___=1?
What do we multiply by to get the identity?
Any number multiplied by its inverse will be the identity.
Identity Matrix
A * A-1= IA-1 *A = I
Any matrix multiplied by its inverse will be the identity matrix.
1001
I
2x2 Identity Matrix
100010001
I
3x3 Identity Matrix
Identity Matrix
AI= A
Just like 5*1 = 5…
1001
IA= A
178
132
1781
32
1001
1781
32
1781
32Or
Ex. 1 Determine whether A and B are inverses.
2413
A
232211
B
Find out if AB = IIf so, then yes they’re inverses.If not, then no they’re not inverses.
Use your calculator.
YES
Ex. 2 Determine whether A and B are inverses.
6332
A
32112
B
YES
Ex. 3 Determine whether A and B are inverses.
4735
A
5734
B
NO
The Inverse of a 2x2 Matrix
dcba
A
AA-1-1==
acbd
bcadacbd
A11
As long as ad-cb As long as ad-cb ==00If ad-cd=0, then the matrix has no If ad-cd=0, then the matrix has no
inverse!!!!inverse!!!!
Ex. 4 Find A-1, if it exists.
7532
A
25371A
25
3715141
AA-1-1==
Ex. 5 Find A-1, if it exists.
0412
A
211410
2410
41
AA-1-1==
Ex. 6 Find A-1, if it exists.
016243
A
Does not exist, because it’s not square.
Ex. 7 Find A-1, if it exists.
326101011
A
142133132
1A
Solving Systems using Solving Systems using Matrices and InversesMatrices and Inverses
Suppose ax = bHow do you solve for x?
We cannot divide matrices, but we can multiply by the inverse.
AX = BAA-1-1 AA-1-1
IX = AA-1-1BX = AA-1-1B
Solving Matrix Equations
Solving a Matrix Equation
3658
1314X
Solve the matrix equation AX=B for the 2x2 matrix X
3658
1314X
Step 1: find AStep 1: find A-1-1
43
111
4311
341AA-1-1==
Step 2: Multiply both sides of the equationStep 2: Multiply both sides of the equationBy ABy A-1-1 on the left on the left
3658
4311
1314
4311
X
3658
4311
1314
4311
X
121524243568
4312121134X
3022
1001X
3022
XYou can check theYou can check the
Solution by multSolution by multAX to see if you get BAX to see if you get B
Ex. Solve
2283
7543X
34214829
X
Assignment
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