# Permutations.. a quick question

• Mar 26th 2006, 01:09 PM
Arbitur
Permutations.. a quick question
How exactly do i go about solving these questions:an example would be fine ;)
• Mar 26th 2006, 01:13 PM
TD!
Example: $\displaystyle \frac{{n!}}{{\left( {n - 1} \right)!}} = \frac{{n\left( {n - 1} \right)!}}{{\left( {n - 1} \right)!}} = n$
• Mar 26th 2006, 01:14 PM
Arbitur
you dont have to use one of my questions but can u do an example with numbers? id understand more
• Mar 26th 2006, 01:16 PM
TD!
Just replace n by a number to see an example, e.g. n = 5:

$\displaystyle \frac{{5!}}{{4!}} = \frac{{5 \cdot 4!}}{{4!}} = 5$
• Mar 26th 2006, 01:20 PM
Arbitur
thanks... im having trouble with this one too.. funny how i get all the hard questions but not these anyways no solving needed here just an example will do( my book sucks)
• Mar 26th 2006, 01:22 PM
TD!
The first one is exactly 5!, no?

Let's take a look at the second one. It starts like 8!, but that is 8*7*6*5*4*3*2*1. But the last part 5*4*3*2*1 isn't there, and that's exactly 5!. So we need 8!, but not the 5!, so 8!/5!. Check:

$\displaystyle \frac{{8!}}{{5!}} = \frac{{8 \cdot 7 \cdot 6 \cdot 5!}}{{5!}} = 8 \cdot 7 \cdot 6$
• Mar 26th 2006, 01:24 PM
Arbitur
so say it was 30*29 whats the answer?
• Mar 26th 2006, 01:25 PM
TD!
Try the same approach as above.