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Math Help - Help on permutations and combinations

  1. #1
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    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
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  2. #2
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    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
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  3. #3
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    Quote Originally Posted by Maccaman View Post
    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
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  4. #4
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    Quote Originally Posted by stathelpplease View Post
    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 .
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  5. #5
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    Quote Originally Posted by mathaddict View Post
    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
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  6. #6
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    Hi stathelpplease

    In that thread, there were discussion about -5!. The answer depends on how you state -5!

    If you mean -(5!), then the answer is -120.
    If you mean (-5)!, then the answer is undefined
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  7. #7
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    Quote Originally Posted by songoku View Post
    Hi stathelpplease

    In that thread, there were discussion about -5!. The answer depends on how you state -5!

    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
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  8. #8
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    Quote Originally Posted by stathelpplease View Post
    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
    There is a difference between 7 - 2! and -2! In the former the symbol is understood to mean subtraction between the two numbers 7 and 2!, in the latter the symbol is understood to be part of the number itself.

    This thread is closed.

    NB: Trolls get banned.
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