We talk about how you feel if someone gives you a positive thing, or if someone takes one away. With the students, we brainstorm on things that are POSITIVE and things that are NEGATIVE. I have a big number line ($^-10$ to $10$, say) above or along the top of my whiteboard. "I believe that adding and subtracting with negative numbers makes sense. Here is a teacher's description of how she explains positive and negative numbers to her classes: You can read the calculation as "Four add negative two, subtract positive five, subtract negative one, add positive seven", and think to yourself "Four, down two, down five, up one, up seven" or equivalent. I add two sandbags (down two), subtract five puffs of hot air (down five), subtract one sandbag (up one), then add seven puffs of hot air (up seven). We can now describe a calculation such as 4 + (-2) - (+5) - (-1) + (+7) in the following way: In this model, we represent positive numbers as 'puffs' of hot air, and negative numbers as sandbags. The first model we offer is the hot air balloon, as seen in the game Up, Down, Flying Around.
There are four possibilities that we need to be able to understand: We hope they will help you to understand what's going on when you might be tempted to use a rule like "Two minuses make a plus". This isn't the most helpful way to think about positive and negative numbers.īelow are some different ways of thinking about adding and subtracting positive and negative numbers. Perhaps you have heard people say "Two minuses make a plus".
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