My problem is when you have a problem like: -7 - -10. I don't understand how to teach them what to do. I found a site that said to have them do the opposite and switch the minus to a plus and the second negative to a positive, so then the answer would be +3. I can see my fifth graders getting so confused. Is there a way that is easier?
I was given this strategy by a co-worker this year. LCO- take the problem -7 - (-10) I always have my students put the last negative number in (). Leave the first number -7 change the sign + and do the opposite for the next number. -7 + 10= 3. They have to write LCO on their papers and then rewrite the problem. If they practice it enough ....they will get it. Hope this helps.
I recently talked to a math specialist and she explained it like this.
On the overhead draw a number line with positive and negative numbers. Then make a character (I liked to cut out a person I printed off the computer and put on an oiverhead so the students can see which way he is facing).
So, take the problem given 7 - (-10)
Have the character start at 7. Since, the next number is a negative number, have your character facing the negative side of the line (to the left)
Okay... so explain to the students that every time you add a number, your character takes a step forward and everytime you subtract, he steps backward. Therefore, since it is subtraction, the character then steps 10 steps backwards landing on #17.
Let me try another problem. -4 -(-6)
The character starts on -4; then, since the next number is a negative, the character faces to the left. Finally, since you are subtracting, the character takes 6 steps backwards landing on 2.
I did this with a fifth grade class and the students really understood. I also had the children act this procedure out and they really got it. I hope this helps and doesn;t confuse you. Let me know if you have any questions.
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.