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Mathzilla Mathzilla is offline
 
Joined: Oct 2007
Posts: 61
Junior Member

Mathzilla
 
Joined: Oct 2007
Posts: 61
Junior Member

Old 05-24-2008, 01:25 PM
 
Clip to ScrapBook #6

I teach them "add the opposite." Leave the first number alone, then handle the problem this way: For this problem, -7 - (-5) =

I make them make 2 marks on their paper (for any subtraction problem.) And we do it as we say Add (first mark) the Opposite (2nd mark). When they say Add, they put a vertical line on their subtraction symbol to make it an addition sign. When they say Opposite, they put a vertical mark on the negative sign in front of the 5. So the problem now looks like this: -7 + (+5) = If that second number had been a positive number, the mark they made would have been a negative sign. Because we're making that second number the opposite of whatever it was originally.

BTW, I have found that most 5th graders developmentally have a difficult time with the concept of manipulating negative numbers. They start to get it in 6th, and finally have the mental facility to work with the problems in 7th.
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