# Why isn't this working? a simple inverse of a 2x2 matrix

1. Oct 3, 2005

### mr_coffee

Okay i have a simple problem, and its not working!
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. Oct 3, 2005

### amcavoy

Last edited: Oct 3, 2005
3. Oct 3, 2005

### mr_coffee

thanks for the responce!! But how did u get a det of -39? if a determinant is defined as
-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. Oct 3, 2005

### amcavoy

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

Last edited: Oct 3, 2005
5. Oct 3, 2005

No!