# Thread: Help on permutations and combinations

1. ## Help on permutations and combinations

I am taking a class this september, and I am trying to read up a bit before it starts. I have a question on the exclamation mark (the "factorial"?).
I think I understand the formula, (I hope ) but when I am actually working with this thing in a calculation, do I have to worry about which comes first? (like I learned last year that multiplication comes before addition) do I do, for example, subtraction before the exclamation mark?

Frank Norman Stein

2. Hi there,

In case you need clarification I'll give you a few examples of the factorial.

$3! = 3*2*1 = 6$
$10! = 10*9*8*7*6*5*4*3*2*1 = 3,628,800$

So, if you had the following question:
Calculate $(3+7+4! -3*2 -3)$. This would be the same as $3+7+4*3*2*1 - 3*2 -3$.
And what does your order of operations tell you about doing subtraction and addition? Is it before or after multiplication?

Hope this helps

3. Originally Posted by Maccaman
Hi there,

In case you need clarification I'll give you a few examples of the factorial.

$3! = 3*2*1 = 6$
$10! = 10*9*8*7*6*5*4*3*2*1 = 3,628,800$

So, if you had the following question:
Calculate $(3+7+4! -3*2 -3)$. This would be the same as $3+7+4*3*2*1 - 3*2 -3$.
And what does your order of operations tell you about doing subtraction and addition? Is it before or after multiplication?

Hope this helps
Ah - I think I see. So since multiplication comes before addition and subtraction, the answer to you problem would be (if I can do my arithmetic without coffee) 25. Is that correct?

Now just to make sure, would 7-2! be equal to 5? (and not 5!)

Frank Norman Stein

Ah - I think I see. So since multiplication comes before addition and subtraction, the answer to you problem would be (if I can do my arithmetic without coffee) 25. Is that correct?

Now just to make sure, would 7-2! be equal to 5? (and not 5!)

Frank Norman Stein
25 is correct .

7-2! = 7 - 2*1

=7-2

=5

5! = 5x4x3x2x1

7-2!=5x4x3x2x1 ??? definitely no .

25 is correct .

7-2! = 7 - 2*1

=7-2

=5

5! = 5x4x3x2x1

7-2!=5x4x3x2x1 ??? definitely no .
Ok that make sense (that 7-2! should equal 5). However, I did a search on factorial on this board and I find this thread:

http://www.mathhelpforum.com/math-he...-what-5-a.html

And they seem to be saying that the minus comes before the factorial?????

Frank Norman Stein

If you mean -(5!), then the answer is -120.
If you mean (-5)!, then the answer is undefined

7. Originally Posted by songoku

If you mean -(5!), then the answer is -120.
If you mean (-5)!, then the answer is undefined

I see. I think I see.
So 7-2! doesn't need parenthesis but -5! does? Do I have it right now?

Frank Norman Stein