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How were you taught negative numbers?

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I find using temperatures can help.

For 7 - 12: "It is 7 degrees and it has become 12 degrees colder. What is the temperature now?"

For -3 + 6: "It is -3 degrees and it warms up by 6 degrees."

For -10 - 4: "It is -10 degrees and it gets 4 degrees colder."

It's a number line in disguise but the context can help.

For 2 - - 3: "What is the difference between a temperature of 2 degrees and -3 degrees?"
(edited 10 years ago)
Reply 21
Original post by TenOfThem
I am still extremely confused by what your are trying to say in the OP


However ... in the spirit of trying to help ... one of the things I do when adding or subtracting is to use hand gestures

moving my hand up or down to indicate + or -

making sure that I ensure my hand is very low when starting or ending on a negative value


That worked for the very basic stuff and quite well indeed. Until I get to things like 5(3)5 - (-3). Their minds go completely blank.
Original post by Boucly
Counting. For 4+2, start at 4 and count 2 more. Similarly for subtraction.

But when subtraction on negative numbers come along, everything just flies out the window.


Say you subtract 2 from a number you would move left on the number line 2 places, so 4-2 would see you move left from 4 to 2. Now start on -4 on the line and subtract 2 again, again you need to move left on the line by 2 places. Getting them to remember what the line looks like might be a bit harder but it's a good starting point.
Original post by Boucly
That worked for the very basic stuff and quite well indeed. Until I get to things like 5(3)5 - (-3). Their minds go completely blank.


For that one really you just need to drill in that two negatives make a positive, it's not particularly mathematical but kids won't understand if you try to explain why that's the case.
Reply 24
Original post by Boucly
That worked for the very basic stuff and quite well indeed. Until I get to things like 5(3)5 - (-3). Their minds go completely blank.


Maybe a sort of 'vector' representation would help? Like, represent numbers as arrows from 0 to where they are on a number line, then they can see how adding a negative number compares to adding a positive number and such like.
I learnt it using a number line at first - my teacher in year 8 wasn't very good so he just gave us a set of rules straight away which confused everyone. :smile:
Original post by Boucly
That worked for the very basic stuff and quite well indeed. Until I get to things like 5(3)5 - (-3). Their minds go completely blank.


Pretty much every student I know can accept that "subtracting a negative number means add"

I never say the "2 negatives make a positive as that causes confusion"
Reply 27
Original post by Mr M
I find using temperatures can help.

For 7 - 12: "It is 7 degrees and it has become 12 degrees colder. What is the temperature now?"

For -3 + 6: "It is -3 degrees and it warms up by 6 degrees."

For -10 - 4: "It is -10 degrees and it gets 4 degrees colder."

It's a number line in disguise but the context can help.

For 2 - - 3: "What is the difference between a temperature of 2 degrees and -3 degrees?"


Ah, it does. Almost everyone can feel the change in temperature. I will try that.
"Left on the numberline, right on the numberline"
"Subtracting makes things go to the left, adding makes them go to the right"
"Subtracting a negative is adding because you can take the two sticks and make a plus out of them"


It seems to have worked and it holds its usage even to degree level. Except now it's imaginary numbers, so they go up and down on the numberline.
Original post by SillyEddy
"Subtracting a negative is adding because you can take the two sticks and make a plus out of them"


Just kill me now.
Original post by Mr M
Just kill me now.

Fabulous. But yeah, that's pretty much how primary school went.
Reply 31
Original post by justanotherposter
For that one really you just need to drill in that two negatives make a positive, it's not particularly mathematical but kids won't understand if you try to explain why that's the case.


That is unfortunately how I had to go about it. To make it worse, I was dealing with 15+ year olds.

Original post by Harantony
I learnt it using a number line at first - my teacher in year 8 wasn't very good so he just gave us a set of rules straight away which confused everyone. :smile:


Right. That is exactly what I want to avoid doing!

Original post by TenOfThem
Pretty much every student I know can accept that "subtracting a negative number means add"

I never say the "2 negatives make a positive as that causes confusion"


Really? May be I've just been rather unlucky. I still have to try to make them understand regardless. I agree that using that rule is not a good idea. But it was the only thing I could think when they didn't get it.

Original post by Stanno
Maybe a sort of 'vector' representation would help? Like, represent numbers as arrows from 0 to where they are on a number line, then they can see how adding a negative number compares to adding a positive number and such like.


Vectors might work. Associating both negation and subtraction with a reversal of direction. It does generalise pretty well. I can try that.
(edited 10 years ago)
Thermometer
Reply 33
I properly got a hang of them when I realised [-3 - 2] was actually [(0 - 3) - 2]. No idea when I learned how to actually do negative subtraction and addition though - I don't remember a time I couldn't so I was always confused why people had such trouble with them.
(edited 10 years ago)
Reply 34
Original post by CJKay
I properly got a hang of them when I realised [-3 - 2] was actually [(0 - 3) - 2].


Ah, the invisible zero. I used that once. But I got into trouble when multiplication came along. He did 2x3=0x2x3 and I screamed, in my head.

Great to know that it does work for some students. I'm learning a lot here, thank you.
Depending on how algebraically competent your students are, a good way of teaching two negatives is as follows:

(1)(22)=0 (-1)(2-2) = 0

(1)(2)+(1)(2)=0 (-1)(2) + (-1)(-2) = 0

(1)(2)=(1)(2) (-1)(-2) = (1)(2)

Intuitively I remember examples being used of debt when I learned it.
Don't know if this will help but in year 8 my teacher said we could imagine it like adding or taking away water from a bath. I still am unsure of myself when using negatives even now though hehheh

If you take hot water from a bath the temperature of the bath decreases (-3-4=-7).
If you take cold water from a bath the temperature of the bath increases (-3--4=1).
If you add hot water to a bath the temperature of the bath increases (-3+4=1).
If you add cold water to a bath the temperature of the bath decreases (-3+-4=-7).

Sorry if I didn't explain it very well :smile: just ask if you think it might be helpful
Original post by DJMayes
Depending on how algebraically competent your students are, a good way of teaching two negatives is as follows:

(1)(22)=0 (-1)(2-2) = 0

(1)(2)+(1)(2)=0 (-1)(2) + (-1)(-2) = 0

(1)(2)=(1)(2) (-1)(-2) = (1)(2)

Intuitively I remember examples being used of debt when I learned it.


Astonishing. Do you really think that someone who struggles with 2 - 3 would have the slightest chance of comprehending this?
Original post by Mr M
Astonishing. Do you really think that someone who struggles with 2 - 3 would have the slightest chance of comprehending this?


I am not a teacher and I am not suggesting it is anywhere near the best or even a good way. However it is a way of showing why the rule actually works, rather than just "here is a rule, follow it" and one that doesn't require contextual hand-waving.
Reply 39
using a number line

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