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Math Help - Alegbraic Expressions

  1. #1
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    Alegbraic Expressions

    I am trying to get an understanding of how these expressions are understood.
    I have an example that I am working on which I require to use Identities to find a value of X.

    The closest example that I can find at the moment from books is this Nested Brackets example;

    7(a - {4 - 5(b - 3a)}) = 7(a - {4 - 5b + 15a})

    = 7(a - 4 + 5b - 15a)

    = 7a - 28 + 35b - 105a

    = 35b - 98a - 28

    So in the above example if I had to work that out starting from the top left hand side, I would follow through as;

    -5b then -5 x -3a = + 15a, so at this point I now have;

    -5b + 15a

    then I would say 7 x a = 7a, and then 7 x 4 = 28, therefore;

    then 7 x 5 = 35b, then 7 x (-15a) = -105a

    so;

    7a - 28 + 35b - 105a = 35b - 98a - 28

    The part I don't understand?

    How does -5b become +5b, and how does +15a become -15a?

    I don't understand it?
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  2. #2
    MHF Contributor Siron's Avatar
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    Re: Alegbraic Expressions

    In general:
    -(-a)=+a
    -(+a)=-a

    Does this answer your question?
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  3. #3
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    Re: Alegbraic Expressions

    Not really I understand what you have written, but in the first two lines of my example the -5b has changed to +5b, and the +15a has become -15a, and based on the above I am not sure how?
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  4. #4
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    Re: Alegbraic Expressions

    The negative sign, same as -1 is multiplied through the brackets.
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  5. #5
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    Re: Alegbraic Expressions

    Quote Originally Posted by pickslides View Post
    The negative sign, same as -1 is multiplied through the brackets.
    So you are saying that what I thought was -{4} is actually -1(4){-5b} worked out as +5b

    If as you say the minus on its own is actually -1 then yes I understand that now
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