# Matrices and permutations

• Jun 2nd 2010, 10:57 AM
doomgaze
Matrices and permutations
I've been going over some review for some placement test I have to take, I've never done anything with Matrices (I think that's the plural for Matrix, haha) and I'm ruuuuusty on solving permutations:
http://img203.imageshack.us/img203/9871/confusek.jpg

I don't think these are particularly difficult, I'm just slow in the subjects..Thanks for anyone who can clarify
• Jun 2nd 2010, 12:08 PM
harish21
Quote:

Originally Posted by doomgaze
I've been going over some review for some placement test I have to take, I've never done anything with Matrices (I think that's the plural for Matrix, haha) and I'm ruuuuusty on solving permutations:
http://img203.imageshack.us/img203/9871/confusek.jpg

I don't think these are particularly difficult, I'm just slow in the subjects..Thanks for anyone who can clarify

$\displaystyle |A|$ means the determinant of a matrix. For a 2x2 matrix, that is a matrix with 2 rows and 2 columns:
$\displaystyle \left[\begin{array}{cc}a&b\\c&d\end{array}\right]$

the determinant is $\displaystyle |A| = ad-bc$.

And if the determinant is zero, the matrix does not have an inverse.
• Jun 2nd 2010, 12:14 PM
harish21
(34) if you are considering the ways in which the committee of 3 people is chosen without order

$\displaystyle 7C3 = \frac{7!}{3!(7-3)!} = \frac{7!}{3! \times 4!}$
• Jun 2nd 2010, 05:11 PM
doomgaze
Quote:

Originally Posted by harish21
(34) if you are considering the ways in which the committee of 3 people is chosen without order

$\displaystyle 7C3 = \frac{7!}{3!(7-3)!} = \frac{7!}{3! \times 4!}$

so you take the total amount of people then divide (total - the # of people per group) ?
• Jun 3rd 2010, 07:41 AM
harish21
Quote:

Originally Posted by doomgaze
so you take the total amount of people then divide (total - the # of people per group) ?

We are NOT taking the total amount/number of people here. The exclamation mark ! that you see after the numbers is called factorial.

7!= 1*2*3....*7
3!= 1*2*3
4!= 1*2*3*4

To know how this formula comes, you need to study permutations and combinations..Google it.

• Jun 4th 2010, 05:41 AM
doomgaze
Quote:

Originally Posted by harish21
We are NOT taking the total amount/number of people here. The exclamation mark ! that you see after the numbers is called factorial.

7!= 1*2*3....*7
3!= 1*2*3
4!= 1*2*3*4

To know how this formula comes, you need to study permutations and combinations..Google it.