Originally Posted by

**thekrown** Okay this doesn't make any sense. I have to find the inverse matrix by using a theorem in my textbook. This theorem uses the adjoint to find the inverse matrix.

Unfortunately it's a 4x4 matrix so it will take me a while to do but there's something that isn't right.

Here is the matrix A

1 3 1 1

2 5 2 2

1 3 8 9

1 3 2 2

My textbook has the answer inverse of A as

-4 3 0 -1

2 -1 0 0

-7 0 -1 8

6 0 1 -7

Okay, now I know how to do this. Let me know if I got this straight.

Step 1: find the matrix of minors, all 16 of them.

Step 2: find the cofactors according to sign charts

Step 3: transpose the cofactor matrix to obtain adjoint of A

Step 4: Find the determinant of the 4x4 matrix

Step 5: 1/det A * adjoint A = inverse matrix of A

If this is correct, can someone help me out finding minors of 4x4 matrices? I can do it for 3x3 but I have difficulty for 4x4, same goes for the determinants. I'm a little confused about the rules but I understand how it works in 3x3 matrices.

Thank you.