Mathmatical Rules, multiplying two negative numbers.

Discussion in 'OT (OFF Topic) Forums' started by Mudster, Apr 24, 2010.

  1. Mudster

    Mudster Hardcore MB Enthusiast

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    Each week the Daily Mail comes with a Weekend supplement imaginatively named "Weekend".

    At the back of this publication is a weekly quiz named "Homework for Grown ups" a quiz which I do each week and I'm lucky to break 5 out of 10 - once managed 8 but mostly get 3 to 4.

    This week one of the questions really fried my noodle.

    It is as follows:-

    -4 x -8 =

    a) 32
    b) -4
    c) -32
    d) 4

    The correct answer is a) 32.

    I've read the mathmatical rules being this and I still don't get it, and I'm reasonably good with maths generally!

    How can multiplying two negatives produce a positive?

    BTW I answered c) -32
     
  2. BlackC55

    BlackC55 Authorised Forum Sponsor Authorised Forum Sponsor

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    I am sure it's -32 as well.
     
  3. LTD

    LTD MB Club Veteran

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    Adding two negatives gives a negative.
    Multiplying two negatives gives a positive.

    It's the rules.
     
  4. npuk

    npuk Hardcore MB Enthusiast

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    Was always taught multiplying two negatives makes a positive so it is correct?
     
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  5. OP
    OP
    Mudster

    Mudster Hardcore MB Enthusiast

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    Well the answer is clearly 32.

    How on earth do you arrive at a positive when multiplying two negatives? I realise it's the rules and I've read the explanations....but the explanations just leave me thinking....eh?
     
  6. npuk

    npuk Hardcore MB Enthusiast

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  7. OP
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    Mudster

    Mudster Hardcore MB Enthusiast

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    Yeah I read that.....and left me thinking.....what? Run that past me again........

    Too much booze tonight, I'll have a rethink tomorrow after I've built the new front steps....
     
  8. renault12ts

    renault12ts MB Club Veteran

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    Two wrongs does make a right!!
     
  9. OP
    OP
    Mudster

    Mudster Hardcore MB Enthusiast

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    Yup that's where my logic stems from.....

    Maybe I should try this next time I have an overdraft, if I multiply my negative balance by a further negative balance, then mathematics states my balance must be in credit!
     
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  10. Spinal

    Spinal Hardcore MB Enthusiast

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    Think of the simplest case.

    1x1 = I'm owed 1 pound from one person, so I have 1 pound

    1x-1 = I owe 1 pound to one person, so I have -1 pounds

    -1x-1 = One person owes me one pound, so I have 1 pounds

    :)
     
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  11. OP
    OP
    Mudster

    Mudster Hardcore MB Enthusiast

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    This also explains why I got about half way through Steven Hawkings "A breif History of Time" at which point the author switched languages to something other than the English I Know.

    I simply cannot get my head around how mulitplying negative 1 by negative one equals positive one.
     
  12. npuk

    npuk Hardcore MB Enthusiast

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    That doesnt make sense they are both the same thing you just changed the wording, why a negative on one and not the other.

    That makes sense.
     
    Last edited: Apr 24, 2010
  13. Dryce

    Dryce MB Club Veteran

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    It's late and I'm bored with nothing else better to do ....

    So here goes.

    -1 + 3 = 2

    Multiply the left hand side by -1 using your answer.

    (-1 x -1) + (-1 x 3) = -1 + (-3) = -4

    Oh dear.

    Now do it the correct way where -1 x -1 = 1 and we get

    (-1 x -1) + (-1 x 3) = 1 + (-3) = -2

    Or you could try looking at division.

    -3 divided by -1 is ? 3

    By inference -3 multiplied by -1 must be the reverse: ie. 3.
     
  14. Spinal

    Spinal Hardcore MB Enthusiast

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    That's addition ;)
    1+(-1)

    In other words

    1x-10 : I owe 10 pounds to one person
    -1x-10 : I'm owed 10 pounds
     
  15. Stratman

    Stratman Hardcore MB Enthusiast

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    If you multiply two positives together you get a positive answer.

    If you change the sign of one of them you also change the sign of the answer: positive multiplied by negative gives a negative answer. Hold that thought.

    You now change the sign again; two negatives are being multiplied. So the sign of the answer also changes.

    Simples.
     
  16. Gridlock

    Gridlock Hardcore MB Enthusiast

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    How about the square root of -1, a number which doesn't exist but which is amazingly helpful in solving numerous equations?

    If you try to *understand* maths in the way you are, you won't get anywhere. The proof of some very simple things is very, very complicated - so just accept that the proof exists :thumb:
     
  17. OP
    OP
    Mudster

    Mudster Hardcore MB Enthusiast

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    I'll carry on in life comfortable not understanding the answer to this questions.


    In the cold light of morning I've realised that I don't actually need to know why it is the case, but it may come in useful in the future if I'm ever faced with having to mulitply two negatives together.

    I'm afraid none of the above explanations make any of this any clearer to me, I also know that it doesn't apply to money and is therefore mostly irrelevant for the forseeable future for me.

    Still fries my noodle though.
     
  18. Gollom

    Gollom Hardcore MB Enthusiast

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    Brain the size of a planet.....
     
  19. OP
    OP
    Mudster

    Mudster Hardcore MB Enthusiast

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    The expalantions sound like Vogon poetry to me.......
     
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  20. crockers

    crockers MB Club Veteran

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    Best you let the accountant do you books ... :D

    Think of double negatives in speech - then you will see why they make a positive...

    I don't not know what I'm doing...regard this as multiplying them = I know what I'm doing.. simples..:D

    Or as you mentioned - applied to money

    He doesn't not owe me £1 = he owes me £1
     
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