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Help with integers! Rules for + and -
Old 02-04-2008, 08:13 PM
 
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Our math book is horrible! I haven't taught this before, new to 5th grade, and the way I learned it is different from the book but I know there are some rules. Anyone have a list of tried and true rules for adding and subtracting negatives and positives?


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I found some :)
Old 02-04-2008, 08:27 PM
 
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Anyone have anything else? This makes sense to me, different than how I learned but I think the kids will understand this way better than the way the TE says to teach them.

ADDITION:

If the signs are the same then you add the two numbers and keep the
sign.

Ex. 6 + 2 = 8 or -6 + -2 = -8

If the signs are different, subtract the two numbers and take the sign
of the larger number.

Ex. -6 + 2 = -4 or 6 + -2 = 4

SUBTRACTION:

Change the sign of the second number, then add the two numbers using the
rules for addition, above.

Ex. 6 - 2 = 6 + (-2) = 4
Ex. -6 - -2 = -6 + (+2) = -4
Ex. -6 - 2 = -6 + (-2) = -8
Ex. 6 - -2 = 6 + (+2) = 8



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Old 02-04-2008, 10:41 PM
 
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Interesting- thanks for posting that!

We do not get into it too much in 5th grade here (not a standard, I do some extra things in math just to help prepare them for 6th grade though, and this is one of them), but I always start out by using a number line and jumping around (like you do in 1st grade sometimes when you learn to add). Like you start at 5, and if you add a -6, you jump backwards 6.... I like those rules you posted. I think it might confuse the heck out of my kids (this group seems easily confused this year), but I might try it anyway
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Old 02-05-2008, 06:27 PM
 
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I'd really recommend you use number lines so you teach kids the concept, not just a rule they don't really understand.

If the number line has negs and positives, you can act out any problem.

6 - -7

Start at the 6, take away moves backward, but the -7 tells you to do the opposite and you land at 13.

-5 + 8

Start at the -5 and add (move forward) 8...end at the 3.

-4 - 9

Start at the -4 and subtract (move backward) 9...you land at the -13.


9 + -4

Start at the 9...adding moves forward, but the - sign tells you to do the opposite. You land at the 5.
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