Pre Algebra Question

[Michelle My Bell]

This is Michelle's daughter. :)

 

I have a math question... I am working on some problems dealing with negative numbers, and I'm a little confused. I can't seem to figure out what to change when adding or subtracting negative numbers.

 

For example, I had this problem today:

 

-14 - (-8)

 

I *thought* I was supposed to change the "minus" sign into a plus sign and then basically add the numbers, and make the sum a negative number. Apparently not.

 

So I looked at the answer key and saw that I was supposed to change the -8 into +8 and change the minus sign to a plus sign. SO, my question is how do you know which number to change to positive? And why do you change it? And will the answer be positive or negative?

 

Thanks. :)

[happi duck]

This is Michelle's daughter. :)

 

I have a math question... I am working on some problems dealing with negative numbers, and I'm a little confused. I can't seem to figure out what to change when adding or subtracting negative numbers.

 

For example, I had this problem today:

 

-14 - (-8)

 

I *thought* I was supposed to change the "minus" sign into a plus sign and then basically add the numbers, and make the sum a negative number. Apparently not.

 

So I looked at the answer key and saw that I was supposed to change the -8 into +8 and change the minus sign to a plus sign. SO, my question is how do you know which number to change to positive? And why do you change it? And will the answer be positive or negative?

 

Thanks. :)

 

There are some posters here that are really great at explaining math via message board...I'm not one of them but I'll give this a try...hopefully this will help a bit and you'll also get some expert advice from other posters.

 

subtracting is adding a negative...8-3=5 can be 8+ (-3)=5

soooo, I tell my kids that you can change subtraction to "plus the opposite" so 8-3=5 becomes "eight plus the opposite of three" or 8+ (-3)=5

 

for your example:

-14 - (-8) it can be rewritten as "negative fourteen plus the opposite of negative eight" or -14+ (+8)=-6

 

I know my terminology isn't "mathy" but I hope that helps some!

[WVNA]

The short answer is that two negatives (such as -(-8)) just become a +8 and the problem reduces to -14+8.

 

Another way to think of it is as (-1)*(-8). That is a +8 and the problem again reduces to -14+8.

 

In the example you gave, the final answer is negative because you are adding 8 to a -14. Whether the final answer is positive or negative depends on what you are adding 8 to. If the original problem was -6-(-8), the answer would be +2.

Edited April 26, 2012 by WVNA

[Trilliums]

This is Michelle's daughter. :)

 

I have a math question... I am working on some problems dealing with negative numbers, and I'm a little confused. I can't seem to figure out what to change when adding or subtracting negative numbers.

 

For example, I had this problem today:

 

-14 - (-8)

 

I *thought* I was supposed to change the "minus" sign into a plus sign and then basically add the numbers, and make the sum a negative number. Apparently not.

So I looked at the answer key and saw that I was supposed to change the -8 into +8 and change the minus sign to a plus sign. SO, my question is how do you know which number to change to positive? And why do you change it? And will the answer be positive or negative?

 

Thanks. :)

 

You can think of it as changing the TWO minus signs (combined) into a plus sign. Then it becomes -14 + 8

14 is larger than 8 so your final answer will be negative (that is the value of 14, disregarding the negative or postive signs, is larger than the value of 8).

 

If you had 14 - (-8), the problem would change to basically 14 + 8 and the answer would be positive.

 

If you had -4 - (-8) your final answer would be positive since 4 is smaller than 8.

 

Good luck!