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Math Help - very confusing!

  1. #1
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    very confusing!

    Could you please tell me why the + sign is changed to a - (minus) in this problem:

    (-5) (x+4y+5)

    = -5x + (-5) (4y) + (-5) (5)
    = -5x - 20y - 25
    they changed the + to - and 20y and 25 became positive instead of negative.

    Does the first number outside of the parenthesis have something to do with this? It only happens when the first number is negative. Its a little confusing because thats not how we were taught to multiply signed numbers.

    I really hope I get through this algebra stuff!!
    Thanks again,
    Rainie
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  2. #2
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    Quote Originally Posted by RainieB View Post
    Could you please tell me why the + sign is changed to a - (minus) in this problem:

    (-5) (x+4y+5)

    = -5x + (-5) (4y) + (-5) (5)
    = -5x - 20y - 25
    they changed the + to - and 20y and 25 became positive instead of negative.

    Does the first number outside of the parenthesis have something to do with this? It only happens when the first number is negative. Its a little confusing because thats not how we were taught to multiply signed numbers.

    I really hope I get through this algebra stuff!!
    Thanks again,
    Rainie

    ur question is a bit confusing.
    can u clarify which + changed to -?
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  3. #3
    MHF Contributor

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    Quote Originally Posted by RainieB View Post
    Could you please tell me why the + sign is changed to a - (minus) in this problem:

    (-5) (x+4y+5)

    = -5x + (-5) (4y) + (-5) (5)
    = -5x - 20y - 25
    they changed the + to - and 20y and 25 became positive instead of negative.
    Did you mean "negative instead of positive"? -20y and -25 are negative.

    Does the first number outside of the parenthesis have something to do with this? It only happens when the first number is negative. Its a little confusing because thats not how we were taught to multiply signed numbers.
    Then how were you taught to multiply signed numbers? I was certainly taught that "negative times positive is negative" so (-5)(4)= -20 and (-5)(5)= -25, exactly what you have.

    I really hope I get through this algebra stuff!!
    Thanks again,
    Rainie[/QUOTE]
    Last edited by HallsofIvy; March 31st 2009 at 09:52 AM.
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  4. #4
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    oh

    This is what I thought it should be :

    (-5) (x+4y+5)

    = -5x + (-5) (4y) + (-5) (5)
    = -5x + -20y + -25

    The minus signs I had originally were subtraction. Yes multiply a negative x positive is negative, but this is what they had in the text:

    =-5 - 20y - 25

    I think they are showing 20y and 25 as positive!
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  5. #5
    MHF Contributor
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    Talking

    Quote Originally Posted by RainieB View Post
    I think they are showing 20y and 25 as positive!
    It sounds as though you are not familiar with negative numbers, since you feel that "+(-20y)" is "positive". To clarify the topic, please try this lesson on negative numbers, which explains, in part, how adding a negative and subtracting a positive are the same thing.
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