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Math Help - Combinatorics/Permut/Solv Eqs Involving Factorial Notation.

  1. #1
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    Combinatorics/Permut/Solv Eqs Involving Factorial Notation.

    Hey any help with the following would be much appreciated:


    Solve for n where:

    P (n,5)=42 * P (n,3)

    based on the fact C(n,r)=n!/(n-r)r! , I got:

    n!/(n-5)!5!=42* n!/(n-3)!3!

    , is this the right step to start? if not, why not? and what would be the proper solution to this problem???
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  2. #2
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    Quote Originally Posted by AGrondin View Post
    Solve for n where:
    P (n,5)=42 * P (n,3) based on the fact C(n,r)=n!/(n-r)r!?
    Problem: P(n,r)=\frac{n!}{(n-r)!}..
    You are using combinations to do permutations.
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  3. #3
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    yes I believe so, any suggestions/help to offer?
    Thanks
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  4. #4
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    This my point, you are using he wrong formula.
    \frac{n!}{(n-5)!}=42\cdot\frac{n!}{(n-3)!}
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  5. #5
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    That would leave me with (n-3)(n-4)=42 leaving me with n=10, n=-3 therefore n=10 ?
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