# Inverting 3 by 3 matricies

1. Dec 20, 2003

### The Divine Zephyr

2. Dec 21, 2003

### Muzza

Isn't it like inverting any other form of matrix? You write up your matrix and the identity matrix like so:

Code (Text):

a b c | 1 0 0
d e f | 0 1 0
g h i | 0 0 1

And perform Gaussian elimination until you reach:

Code (Text):

1 0 0 | x y z
0 1 0 | p q w
0 0 1 | r t u

Then the matrix to the right of the |-signs is the inverse you're looking for.

Last edited: Dec 21, 2003
3. Dec 22, 2003

### HallsofIvy

Staff Emeritus
Yep. There are other methods (cofactors divided by determinant) but row-reduction is the simplest.

4. Dec 22, 2003

### The Divine Zephyr

Awesome! But.. umm... what is Gaussian elimination?

5. Dec 23, 2003

### HallsofIvy

Staff Emeritus
If you are going to ask questions about Linear Algebra it would be a good idea to read at least the first few chapters of a textbook on linear algebra!

"Gaussian Elimination" is basically the method of "elimination of variables" to solve a system of equations- multiply one equation by a number, add to another in order to eliminate one of the variables. It is often used specifically to denote the same thing applied to matrices of coefficients of the equations.

6. Dec 23, 2003

### The Divine Zephyr

Oh, that? I get it. Thank you.

7. Jan 15, 2004

### franz32

hello.

It would also be better to practice solving for the inverse of a matrix. And, it would be easier for you if you know know the topic by heart. =)

Of course, not all matrices have an inverse, or what we call a nonsingular matrix. This is very "special" later in the topic.
One example is the use of determinants.

8. Feb 24, 2004

### phantomAI

Yeah if the determinant of a matrix is zero, it does not contain an inverse and is a nonsingular matrix.

9. Feb 25, 2004

Staff Emeritus
You've got singular and nonsingular backward. Singular matrices have zero determinants and don't have inverses, because in computing the inverse you divide by the determinant (even if you don't think you do!) and dividing by zero is a "singular" mathematical operation, i.e. not defined.

Nonsingular matrices do have inverses, and necessarily then they have nonzero determinants.

10. Mar 2, 2004

### Singularity

I believe the formula for the inverse of a nxn matrix is

inverse of A = 1/(det(A)) * adj(A)

i speak under correction. we did this last year and i have forgotten most of last year's stuff during the holidays