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Math Help - Sove simple equation doubt

  1. #1
    Newbie FernandoBasso's Avatar
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    Sove simple equation doubt

    I'm solving this:
    Code:
    3  7 * (1 - 2x) = 5  (x + 9)
    And am trying this way:
    Code:
    3- 7 * (1 - 2x) = 5 - (x + 9)
    3 - 7 ..?? ??   = 5 - x - 9)
    What should be done in the part I put those question marks?

    I'm really lost.

    Thanks in advance for any help.
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  2. #2
    MHF Contributor Siron's Avatar
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    Re: Sove simple equation doubt

    Do you know the distributive law?
    In general: a\cdot (b+c)=a\cdot b+a\cdot c

    Use it to expand -7\cdot (1-2x)=...
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  3. #3
    Newbie FernandoBasso's Avatar
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    Re: Sove simple equation doubt

    Thanks for the reply.

    So, I should do:

    3 - 7 . (1 - 2x) = 5 - (x + 9)
    3 - 7 . 1 - 7 . 2x = 5 - x - 9)
    3 -  (-7) - 14x = 5 - x - 9
     -10x = ...

    I'm lost...
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  4. #4
    MHF Contributor Siron's Avatar
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    Re: Sove simple equation doubt

    You have to pay attention with the signs:
    3-7\cdot (1-2x)=5-(x+9)
    \Leftrightarrow 3-7\cdot 1 -7\cdot (-2x)=5-x-9
    \Leftrightarrow 3-7+14x=5-x-9
    \Leftrightarrow 3-7-5+9=-x-14x
    \Leftrightarrow 0=-15x
    \Leftrightarrow x=0
    Last edited by Siron; September 21st 2011 at 08:02 AM.
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  5. #5
    Newbie FernandoBasso's Avatar
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    Re: Sove simple equation doubt

    I was doing:
    Code:
    3 - 7 * (1 - 2x) = 5 - (x + 9)
    3 - 7 * 1 - 7 * (-2x) = ...
     3 - (-7) - 14x = ...
    When I should be doing:
    Code:
    3 - 7 * (1 - 2x) = 5 - (x + 9)
    3 - 7 * 1 - 7 * (-2x) = ...
     3 - 7 - 14x = ...
    Why don't I put -7 inside parentheses?
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  6. #6
    MHF Contributor Siron's Avatar
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    Re: Sove simple equation doubt

    If you expand:
    7(1-2x)=7-14x
    And the LHS is:
    3-[7(1-2x)]=3-(7-14x)=3-7+14x

    If you expand:
    -7(1-2x)=-7+14x
    Now, the LHS is:
    3+[-7(1-2x)]=3-7+14x
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