# Help on permutations and combinations

• Aug 15th 2009, 04:45 AM
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 (Wink)) 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
• Aug 15th 2009, 05:47 AM
Maccaman
Hi there,

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

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

So, if you had the following question:
Calculate \$\displaystyle (3+7+4! -3*2 -3) \$. This would be the same as \$\displaystyle 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
• Aug 15th 2009, 07:04 AM
Quote:

Originally Posted by Maccaman
Hi there,

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

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

So, if you had the following question:
Calculate \$\displaystyle (3+7+4! -3*2 -3) \$. This would be the same as \$\displaystyle 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
• Aug 15th 2009, 08:10 AM
Quote:

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 .
• Aug 15th 2009, 09:49 AM
Quote:

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
• Aug 15th 2009, 10:13 AM
songoku

If you mean -(5!), then the answer is -120.
If you mean (-5)!, then the answer is undefined
• Aug 15th 2009, 12:56 PM
Quote:

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
• Aug 15th 2009, 02:14 PM
mr fantastic
Quote: