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Math Help - commutative, associative

  1. #1
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    commutative, associative

    p*q = p+q-pq

    1. determine whether * is commutative
    2. determine whether * is associative.


    Ok im not sure i understand the question. Do i just work out the answers from the right side of the equation or both sides?
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  2. #2
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    Quote Originally Posted by djmccabie View Post
    p*q = p+q-pq

    1. determine whether * is commutative
    2. determine whether * is associative.


    Ok im not sure i understand the question. Do i just work out the answers from the right side of the equation or both sides?
    It is not an equation, it is a definition of the binary operation "*" which is not the usual product.

    CB
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  3. #3
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    ahh, so i work out commutative and associative for p+q-pq ??

    and did i post this in the wrong place?
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  4. #4
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    Quote Originally Posted by djmccabie View Post
    ahh, so i work out commutative and associative for p+q-pq ??

    and did i post this in the wrong place?
    1. Yes.

    2. Probably, but I will move it, on second thoughts it could be OK where it is.

    CB
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  5. #5
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    ok so i would write -

    1.

    p+q-pq does not equal p-q+pq

    therefore not commutative?


    and then similarly to associative?

    (p+q)-pq does not equal p-(q+pq)


    your help is much appreciated
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  6. #6
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    Quote Originally Posted by djmccabie View Post
    ok so i would write -

    1.

    p+q-pq does not equal p-q+pq

    therefore not commutative?
    p*q = p+q-pq

    q*p = q+p-qp

    but normal addition and multiplication are commutative so:

    q*p = q+p-qp = p+q-pq = p*q

    so "*" is commutative.

    CB
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  7. #7
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    Quote Originally Posted by djmccabie View Post


    and then similarly to associative?

    (p+q)-pq does not equal p-(q+pq)


    your help is much appreciated
    To determine if "*" is associative you need to determine if:

    p*(q*r) = (p*q)*r

    CB
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  8. #8
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    thanks a lot i understand it now, did i do the associativity right?
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  9. #9
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    Quote Originally Posted by djmccabie View Post
    thanks a lot i understand it now, did i do the associativity right?
    No see the earlier post that crossed yours in the aether

    CB
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