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Why wouldn't -7 + -8 = a positive 15? You always hear people say a (-) + (-) = (+) a negative plus a negative equals a positive. Why isn't it in this case? It just doesn't make sense to me?!?!?!?!? Please give me some tips or the logic behind it. Thanks a bunch!!

 May 22, 2014

Best Answer 

 #2
avatar+99388 
+10

Good question!

Two minus make a plus ONLY

1) if they are so close together that you can put a circle around them and there is nothing else in  the circle.

Think of it like this if you want

Negative numbers are cold numbers.  Like -30degrees would be very cold wouldn't it?

and Positive numbers are hot numbers.  An oven might be 180degrees. There is an invisible + in front of the 180.

So 15--2 means you are taking away cold - so it must get hotter

15--2=15+2=17

2) If you are multiplying or dividing

--------------------------------------------------------------------

NOW if you are adding or subtracting positive and negative numbers then you need to think about a number line.

10+3 means you start at 10 and go 3 places in the positive direction = 13

10-3 means you start at 10 and go 3 places in the negative direction = 7

Just continue this method with trickier negative numbers

-3-8 means you start at -3 and go 8 places in the negative direction = -11

OR to put it another way.  It is -3 degrees and it gets 8 degrees COLDER = -11 degrees

So lets look at your question

-7+-8 = If you add cold it gets colder so the question becomes

-7-8     Now if it is -7 degrees that pretty cold and you take away 8 degrees, it is going to get even colder! So the answer must be more negative.  

Start at -7 on the number line and go 8 places in the negative direction.  The answer is -15

I don't know why I am writing all this I am sure that I have done it before as a post to keep.  Not to worry.

Here is a number line that might help you. You'll have to add the negative numbers in yourself.

Here is a site that might also help.   

http://www.mathsisfun.com/positive-negative-integers.html

 May 22, 2014
 #1
avatar+98197 
+5

Think about this.......If the temperature was -8 and then it dropped another 7 degees....would it have risen 15 degrees??  No.....it would have dropped -8 + -7 = -15 degrees !!

You're getting this confused with a negative times a negative, which = a positive.

This one is harder to see......suppose you have lost 7 dollars for the last 8 weeks......then in 8 more weeks, you will have 56 dollars less...(8) times (-7). But 8 weeks ago, (-8), you had (-8) times (-7) = 56 dollars more. Does that make sense??

So, adding two negatives produces a negative, but multiplying two negatives produces a positive.

Hope that helps......

 May 22, 2014
 #2
avatar+99388 
+10
Best Answer

Good question!

Two minus make a plus ONLY

1) if they are so close together that you can put a circle around them and there is nothing else in  the circle.

Think of it like this if you want

Negative numbers are cold numbers.  Like -30degrees would be very cold wouldn't it?

and Positive numbers are hot numbers.  An oven might be 180degrees. There is an invisible + in front of the 180.

So 15--2 means you are taking away cold - so it must get hotter

15--2=15+2=17

2) If you are multiplying or dividing

--------------------------------------------------------------------

NOW if you are adding or subtracting positive and negative numbers then you need to think about a number line.

10+3 means you start at 10 and go 3 places in the positive direction = 13

10-3 means you start at 10 and go 3 places in the negative direction = 7

Just continue this method with trickier negative numbers

-3-8 means you start at -3 and go 8 places in the negative direction = -11

OR to put it another way.  It is -3 degrees and it gets 8 degrees COLDER = -11 degrees

So lets look at your question

-7+-8 = If you add cold it gets colder so the question becomes

-7-8     Now if it is -7 degrees that pretty cold and you take away 8 degrees, it is going to get even colder! So the answer must be more negative.  

Start at -7 on the number line and go 8 places in the negative direction.  The answer is -15

I don't know why I am writing all this I am sure that I have done it before as a post to keep.  Not to worry.

Here is a number line that might help you. You'll have to add the negative numbers in yourself.

Here is a site that might also help.   

http://www.mathsisfun.com/positive-negative-integers.html

Melody May 22, 2014
 #3
avatar+99388 
0
 Jun 1, 2014
edited by Melody  Oct 24, 2017
edited by Melody  Oct 24, 2017

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