inverse & identity matrices section 4.5. objectives you will write the identity matrix for any...
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Inverse & IdentityMatrices
Section 4.5
Objectives
You will
• write the identity matrix for any square matrix
• find the inverse of a 2 x 2 matrix
The Identity Matrix IThe identity matrix is a
square matrix with 1’s on the principal diagonal. All other elements are 0
It’s the only matrix that’s commutative
A • I = I • A = A
1 2 4
0 9 8
7 5 3
1 0 0
0 1 0
0 0 1
=1 2 4
0 9 8
7 5 3
The identity matrix will always be the same dimension as the other matrix.
1 0
0 1
If you multiply the identity matrix is by a 2nd matrix, the product is equal to the second matrix
(it’s like multiplying by 1)
Principal Diagonal
Inverse Matrix A-1
n • 1/n = 1/n • n = 1 is the multiplicative inverse (for real numbers ≠ 0)
A• A-1 = A-1 • A = 1 for matrices if A-1 exists
A 2x2 matrix will have an inverse if its determinant ≠ 0
a b
c d
has the determinanta b
c d= ad – cb
If ad = cb, the matrix does not have an inverse
Finding A-1
Find the determinant of the matrix
Switch A1,1 & A 2, 2
a b
c d= ad – cb
If ad – cb = 0, you’re done—no inverse exists
a b
c d
If ad ≠ cb the matrix has an inverse
1
ad cb
Put “1” over the value of the determinant
If A =
a b
c d
d b
c a
Then change the signs on A1,2 & A2,1 & put the fraction on the left
d b
c a
1
ad cb = A-1
find M-1
det of M =
2 5
0 7
If M =
2 5
0 7
= 14 – 0 = 14
The fraction to use for the inverse is1
14
Change the matrix to inverse form: 7 5
0 2
(7 & 2 changed places, the sign on the –5
changed to positive & 0 stayed 0)
Set up the inverse: 1
14
7 5
0 2
= M-1
Gotta get in some practice!
• Find A-1 if A = 4 3
2 1
(click to check)• Find B-1 if B =
1 2 4
3 0 5
(click to check)
• Find C-1 if C = (click to check)2 6
1 3
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