Originally Posted by

**Adrian** So for cofactor expansion I should find the detA to start off, but I'm not sure how to find the determinant of a 4x4 matrix?

-1 -4 2 1

2 -1 7 9

-1 1 3 1

1 -2 1-4

Do I find the determinants of each of the 3x3 matrices I am left with after going through each of the numbers in column 1?

If so, I should start at the -1 in Row 1 Column 1

That leaves me with:

-1 7 9

1 3 1

-2 1 -4

and the determinant is -1(-12-2) + -7(-4+2) + 9(1+6) = 91

Then move on to the 2nd number in column 1 of the 4x4 matrix?

That leaves me with:

-4 2 1

1 3 1

-2 1 -4

And find the determinant of that.

Then do the same thing with the -1 in Row 2 and the 1 in Row 4 of column 1 of the 4x4 matrix.

If I am correct so far, after going through all 4 numbers in Column 1 would I add the results of their determinants together? And this would be my determinant of the 4x4 matrix?

Thanks, but if I can learn to find the determinant of a 4x4 matrix then I should be able to deal with this problem.