# Matrix doing a cofactor expansion

• Aug 31st 2009, 09:06 PM
lin.13579
Matrix doing a cofactor expansion
Evaluate the determinant of the following matrix A =
$
\left|\begin{array}{ccccc}2 & 1 & -3 & -1 & 2 \\ 0 & -1 & 0 & 3 & 1 \\ 0 & 2 & 0 & 0 & 1 \\ 0 & 0 & 1 & 0 & 2 \\ 0 & 3 & 0 & 3 & 2 \end{array}\right|
$

by doing a co-factor expansion.

Please expalin how to do it, thank you!
this is the exercise on my task
• Aug 31st 2009, 10:10 PM
eXist
If you have:

$\begin{array}{lcr}
\mbox a & b & c \\
\mbox d & e & f \\
\mbox g & h & i\end{array}
$

The cofactor for $a$ is the determinant of:

$\begin{array}{lcr}
\mbox e & f \\
\mbox h & i \end{array}
$

The cofactor for $b$ is negative(-) of the determinant of:

$\begin{array}{lcr}
\mbox d & f \\
\mbox g & i \end{array}
$

The cofactor for $c$ is the determinant of:

$\begin{array}{lcr}
\mbox d & e \\
\mbox g & h \end{array}
$
• Sep 1st 2009, 07:38 AM
HallsofIvy
Why are you being asked to do a "co-factor expansion" if you don't know what that is?

I would recommend starting your co-factor expansion using the first column. That way you just have 2 times $\left|\begin{array}{cccc}-1 & 0 & 3 & 1 \\ 2 & 0 & 0 & 1 \\ 0 & 1 & 0 & 2 \\ 3 & 0 & 3 & 2\end{array}\right|$ and expand that determinant on the second column. Do you see why?
• Sep 2nd 2009, 05:10 PM
lin.13579
Quote:

Originally Posted by HallsofIvy
Why are you being asked to do a "co-factor expansion" if you don't know what that is?

I would recommend starting your co-factor expansion using the first column. That way you just have 2 times $\left|\begin{array}{cccc}-1 & 0 & 3 & 1 \\ 2 & 0 & 0 & 1 \\ 0 & 1 & 0 & 2 \\ 3 & 0 & 3 & 2\end{array}\right|$ and expand that determinant on the second column. Do you see why?

I am sorry, I am not good on maths , can you explain more please !!!
I don't understand what is "just have 2 times" mean?
Is it mean 2 times every number 1 & 0 & 3 & 1 \\ 2 & 0 & 0 & 1 \\ 0 & 1 & 0 & 2 \\ 3 & 0 & 3 & 2 to evaluate matrix A?