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Math Help - -7y = 0

  1. #1
    Junior Member Euclid Alexandria's Avatar
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    Question -7y = 0

    I came out with the right answer, but did I work this out correctly?

    -7y = 0
    -7y/-7 = 0/-7
    -y = 0
    -y/-y = 0/-y
    y=0
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  2. #2
    Super Member
    earboth's Avatar
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    Quote Originally Posted by Euclid Alexandria
    I came out with the right answer, but did I work this out correctly?

    -7y = 0
    -7y/-7 = 0/-7
    -y = 0
    -y/-y = 0/-y
    y=0
    Hello,

    you have made only a minor mistake:
    -7y/-7 = 0/-7
    y = 0

    because -7y/-7 = +y the last two lines aren't necessary (and slightly wrong).

    Greetings

    EB
    Last edited by earboth; March 3rd 2006 at 07:49 AM.
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  3. #3
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    also going from
    -y = 0
    and you then said
    -y/-y = 0/-y
    this is not equivalent to:
    y=0
    as you said.

    you had -y=0
    so what you want to do here to get the solution is divide both sides by -1
    -y/-1 = 0/-1
    y=0

    since it turns out y=0
    before you were trying to say -y/-y which is 0/0 which is undefined.
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  4. #4
    Junior Member Euclid Alexandria's Avatar
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    Thumbs up

    Thanks, guys. I couldn't remember the rule for getting a negative variable to be positive. You always divide both sides by -1?
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  5. #5
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    you could either divide by -1, multiply by -1, the effect is the same. you don't tend to think of it as "multiplying/dividing by -1" though. you just change the sign of both sides. for instance if
    -x = -4
    then
    x = 4
    you are simply changing the sign of both sides. you don't really need to explain yourself when doing that
    likewise if you had
    -y=17
    then changing the signs of each side gives:
    y=-17
    however multiplying through by -1 is a more common way of thinking about it than dividing through by -1.
    Last edited by Aradesh; March 4th 2006 at 06:04 PM.
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  6. #6
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by Aradesh
    you are simply changing the sign of both sides. you don't really need to explain yourself when doing that
    Actually I've found that the most common, and difficult to find, mistakes are due to sign errors. I recommend (though don't force) my students to show every step for that reason, even if it seems trivial to do so. I find it makes for good technique.

    -Dan
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