when do two negatives not make positive?

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PaulineDent
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Just can't get my head round this to support my grand daughter. Yr 8 'two minuses make a plus' find example of when this rule does work and when it doesn't . thanks
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RichE
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(Original post by PaulineDent)
Just can't get my head round this to support my grand daughter. Yr 8 'two minuses make a plus' find example of when this rule does work and when it doesn't . thanks
What does "make" here mean - times?

Multiplying two negatives always gives a positive.
Dividing two negatives always gives a positive.
Adding two negatives always gives a negative.
One negative subtracted from another can leave a positive or negative answer or 0.
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PaulineDent
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(Original post by RichE)
What does "make" here mean - times?

Multiplying two negatives always gives a positive.
Dividing two negatives always gives a positive.
Adding two negatives always gives a negative.
One negative subtracted from another can leave a positive or negative answer or 0.
Thanks
Sorry I hoped to avoid typing the whole question but it would have been
more helpful to you if I had.
it was a question set by the school

'A child says is calculating with negative number. he says " two minuses make a
plus" His teacher tells him that this rule is inaccurate and unhelpful.
a) give an example of a calculation and its solution where this rule DOES work
b) give and example of a calculation and its solution where this rule DOES NOT
work.
c) explain how you worked out a)
d) explain how you worked out b)

I am fine with examples of where rule works. but only situation I can think of
where it does not work is adding two negatives. e.g -3 + -5 = -8
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atsruser
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(Original post by PaulineDent)
I am fine with examples of where rule works. but only situation I can think of
where it does not work is adding two negatives. e.g -3 + -5 = -8
Two other possibilities:

1) Subtraction:

-10 -(-5) = -10 + 5 = -5

2) Taking powers:

(-2)^{-3}=\dfrac{1}{(-2)^3} = \dfrac{1}{-8} = -\dfrac{1}{8}
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rickH123
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(Original post by PaulineDent)
Just can't get my head round this to support my grand daughter. Yr 8 'two minuses make a plus' find example of when this rule does work and when it doesn't . thanks
well, think of this like this considering she's ur grand daughter.

Bad things happening to good people (negative and positive) is bad (negative answer)

Good things happening to bad people (positive and negative) is bad(negative answer)

bag things happening to bad people (negative and negative) is good (positive answer)

Good things happening to good people (good and good) is good (positive answer)

It's one way which i tackled this problem. This form of analogy can work both for adding AND multiplying negatives and positives.
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PaulineDent
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(Original post by RichE)
What does "make" here mean - times?

Multiplying two negatives always gives a positive.
Dividing two negatives always gives a positive.
Adding two negatives always gives a negative.
One negative subtracted from another can leave a positive or negative answer or 0.
please could you give me an example for the last situation on the list, i.e. a positive, negative and zero answer for a calculation.
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Kiritsugu
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(Original post by rickH123)
well, think of this like this considering she's ur grand daughter.

Bad things happening to good people (negative and positive) is bad (negative answer)

Good things happening to bad people (positive and negative) is bad(negative answer)

bag things happening to bad people (negative and negative) is good (positive answer)

Good things happening to good people (good and good) is good (positive answer)

It's one way which i tackled this problem. This form of analogy can work both for adding AND multiplying negatives and positives.
lmao nice one
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NoPunInThisName
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HIV.
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RichE
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(Original post by PaulineDent)
please could you give me an example for the last situation on the list, i.e. a positive, negative and zero answer for a calculation.
(-3) - (-2) = ?
(-3) - (-3) =?
(-3) - (-4) = ?
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PaulineDent
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(Original post by RichE)
(-3) - (-2) = ?
(-3) - (-3) =?
(-3) - (-4) = ?
. It's looks so simple now I dont know how I missed it!
Thank you very much for taking the time to help. It is very much appreciated.
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PaulineDent
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(Original post by rickH123)
well, think of this like this considering she's ur grand daughter.

Bad things happening to good people (negative and positive) is bad (negative answer)

Good things happening to bad people (positive and negative) is bad(negative answer)

bag things happening to bad people (negative and negative) is good (positive answer)

Good things happening to good people (good and good) is good (positive answer)

It's one way which i tackled this problem. This form of analogy can work both for adding AND multiplying negatives and positives.
thanks...............we love this idea ..............very helpful
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rickH123
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(Original post by PaulineDent)
thanks...............we love this idea ..............very helpful
Hope i helped :P
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