- #1

- 1,629

- 1

I have the following:

Find a 2x2 matrix

A =

-5 3

2 -9

A-1 =

-3/13 -1/13

2/39 -5/39

A*A-1 = I

1 0

0 1

but its saying the inverse i got is wrong! how is this?

det i got was 1/39

- Thread starter mr_coffee
- Start date

- #1

- 1,629

- 1

I have the following:

Find a 2x2 matrix

A =

-5 3

2 -9

A-1 =

-3/13 -1/13

2/39 -5/39

A*A-1 = I

1 0

0 1

but its saying the inverse i got is wrong! how is this?

det i got was 1/39

- #2

- 665

- 0

Your determinant is actually 39 (not 1/39). Divide the adjoint matrix by that and you should get your answer (provided you calculated the adjoint correctly).mr_coffee said:

I have the following:

Find a 2x2 matrix

A =

-5 3

2 -9

A-1 =

-3/13 -1/13

2/39 -5/39

A*A-1 = I

1 0

0 1

but its saying the inverse i got is wrong! how is this?

det i got was 1/39

Last edited:

- #3

- 1,629

- 1

1/(ad-bc)?

so if i had

-5 3

2 -9

(-5)(-9) -(3)(2) = 45 - 6 = 39, and its under 1, so 1/39?

I also tried ur answer, when u say divide the adjoint matrix, do u mean divide the matrix after switching a and d, and negating b and c? I tried that and it was also wrong with ur det

- #4

- 665

- 0

The determinant of a 2x2 matrix is ad-bc. I meant to say 39, not -39.mr_coffee said:

1/(ad-bc)?

so if i had

-5 3

2 -9

(-5)(-9) -(3)(2) = 45 - 6 = 39, and its under 1, so 1/39?

I also tried ur answer, when u say divide the adjoint matrix, do u mean divide the matrix after switching a and d, and negating b and c? I tried that and it was also wrong with ur det

Last edited:

- #5

TD

Homework Helper

- 1,022

- 0

No!mr_coffee said:thanks for the responce!! But how did u get a det of -39? if a determinant is defined as

1/(ad-bc)?

The inverse is 1/det(A) * adj(A) where 1/det(A) = 1/(ad-bc) so det(A) = ad-bc and not 1/(ad-bc).

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