Neat trick for multiplying/dividing integers

Discussion in 'Sixth Grade' started by Teacheroo, Sep 9, 2007.

  1. Teacheroo

    Teacheroo Companion

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    Sep 9, 2007

    I just learned this trick and thought I'd share it. I had tons of light bulb moments in my class when I shared it with them! Hopefuly it makes sense on here. :)


    Good Guy (+) Comes to town (+) Good thing (+)
    Bad Guy (-) x Leaves town (-) = Bad thing (-)
    ____________ _______________ _______________

    + x + = +

    - x + = -

    + x - = -

    - x - = +


    So...If a good guy comes to your town it's a good thing (+)
    If a bad guy comes to your town it's a bad thing (-)
    If a good guy leaves your town it's a bad thing (-)
    If a bad guy leaves your town it's a good thing (+)

    A little easier for my kids than just memorizing the rules.


    ETA. Well, this didn't work like I meant for it to. I couldn't get my +/- to line up under the columns. if you have questions about it just ask. :)
     
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  3. JaimeMarie

    JaimeMarie Moderator

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    Sep 9, 2007

    That is way confusing for me. Sorry.
     
  4. jazzminjoy

    jazzminjoy Comrade

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    Sep 9, 2007

    It is about multiplication with signs. It is a way to remember that a negative times a negative is a positive, and that a negative times a positive is a negative (and so on). I think it is pretty straight forward. Stories help people learn and remember. Nice idea.
     
  5. smalltowngal

    smalltowngal Multitudinous

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    Sep 9, 2007

    I get the story aspect of it, but in your first explanation you said a bad guy leaves your town its a bad thing (-x-=-) but then in the story you said if a bad guy leaves your town its a good thing (-x-=+)
     
  6. JaimeMarie

    JaimeMarie Moderator

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    Sep 9, 2007

    That is why I was confused too.

    And what happens when you have a bad guy times a bad guy? that is a postive?
     
  7. Aliceacc

    Aliceacc Multitudinous

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    Sep 9, 2007

    I told a version of that to my 9th graders on Friday. Only in the version I've heard it's "when a bad thing happens to a bad person..." Certainly not a philosophy to live by, but the kids find it cute:

    When a good thing happens to a good person, that's good.
    When a bad thing happens to a bad person, that's good.
    When a good thing happens to a bad person, that's bad.
    When a bad thing happens to a good person, that's bad.

    Disclaimer: this is not my philosophy, nor is it the one my school espouses. But it's a cutesy thing to show them.
     
  8. jaruby

    jaruby Companion

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    Sep 9, 2007

    Its supposed to be a table and the x and = sign on the top are just seperating the rows, it is not an equation. Her formatting is off so it smushed everything together.
     
  9. MissHunny

    MissHunny Comrade

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    WOW! i teach second grade but this makes great sense to me!! Too bad I won't need to use it this year!
     
  10. lupin43

    lupin43 Companion

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    Sep 10, 2007

    I use a similar analogy. Tigers and Bears. Both are the same strength so if put in the same cage - they both die.

    Addition
    bears(+) + bears = more bears (add, pos.)
    tigers(-) + tigers = more tigers (add, neg.)
    tigers + beats = who wins, by how many (subtract, use sign of large values)

    Subtraction is hard to explain with just words, but it boils down to adding the opposite.

    Multiplication

    groups of animals.

    4*3 (4 groups of 3 bears) = 12
    4 * (-3) (4 groups of 3 tigers) = -12
    -4 *-3 (TAKE OUT 4 groups of 3 tigers, leaves bears) = 12
    -4 * 3 (take out 4 groups of 3 bears, leaves tigers) = -12

    The kids really get into the idea of dead animals all over the place. Is it PC, probably not, but they certainly remember it.

    ***Disclaimer****
    No animals were harmed in the creation of this post or in the implementation of this lesson plan.
     
  11. Upsadaisy

    Upsadaisy Moderator

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    Sep 10, 2007

    Isn't it easier just to remember, 'sign's the same, answer is positive'?
     
  12. smalltowngal

    smalltowngal Multitudinous

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    Sep 10, 2007

    That's great Upsadaisy!! I never could really remember signs...of course, I'm sure it would have been easier had I been told this. :)
     
  13. Upsadaisy

    Upsadaisy Moderator

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    Sep 10, 2007

    Remember, that's only when multiplying and dividing, though. When we work on integers, I repeat these rules soooooo many times that they become like a mantra!
     
  14. MrsC

    MrsC Multitudinous

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    That's what I do!
     
  15. Teacheroo

    Teacheroo Companion

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    Sep 12, 2007

    yeah, sorry everyone. It was supposed to be a table. The top part was supposed to be column headings.
    Basically its this:

    If a good guy (+) comes to your town(+) its a good thing (+)
    If a bad guy (-) comes to your town (+) its a bad thing (-)
    If a good guy (+) leaves your town (-) its a bad thing (-)
    If a bad guy (-) leaves your town (-) its a good thing (+)

    Does that make more sense?
     

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