Log in

View Full Version : Algebra 2 finding the determinant of a four by four matrices using ellimination.


aseperatepeace
Nov 24, 2009, 08:55 AM
How do you find the determinant of this four by four matrices by using ellimination?

MY MATRICES:

0 -2 3 -4
-5 -6 7 -8
9 -10 11 -12
13 -14 -15 16

P.S I know how to find the determinant of a three by three and a two by two easily using ellimination, but struggle with a four by four.

THANK YOU FOR YOUR TIME!

galactus
Nov 26, 2009, 09:00 AM
It is tedious to do these things by hand. In my opinion, it has become rather anachronistic to do these manually when calculators will do them lickety-split. The important thing is knowing when to use a det. Let a calculator do the grunt work. One can use cofactors. Or, using row reduction methods, turn the matrix into a upper or lower triangular matrix and bring out common factors to multiply.

I will tell you the determinant is -2128.

There is something to shoot for if you want to give it a go.

There are some det laws that are helpful to know.

Here is an example of what I mean.

A=\begin{bmatrix}0&1&5\\3&-6&9\\2&6&1\end{bmatrix}

Interchange first and second rows:

-\begin{bmatrix}3&-6&9\\0&1&5\\2&6&1\end{bmatrix}

Remove a common factor of 3 from the first row:

-3\begin{bmatrix}1&-2&3\\0&1&5\\2&6&1\end{bmatrix}

-2 times first row added to third row:

-3\begin{bmatrix}1&-2&3\\0&1&5\\0&10&-5\end{bmatrix}

-10 times second row added to third row:

-3\begin{bmatrix}1&-2&3\\0&1&5\\0&0&-55\end{bmatrix}

Remove a common factor of -55 from the last row:

(-3)(-55)\begin{bmatrix}1&-2&3\\0&1&5\\0&0&1\end{bmatrix}

(-3)(-55)(1)=\fbox{165}