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I've never had to teach this, except remedially, where honestly, time is a major issue. A teacher in my building has students record someone walking backwards down the hallway, then plays the video back in reverse. The student appears to walk forwards down the hallway, more convincingly than you'd expect.[1]

From there, you can show that -(-1) = 1. If the students have any command of factoring, you can use that to handwavingly show, for example, that -2 ✕ -3 = -1 ✕ 2 ✕ -1 ✕ 3 = -(-6)) = 6.

[1]She actually has them walk forwards, and hold signs - it's more involved, and covers all four cases for multiplying negatives and positives, but we'll gloss over that for the moment.

edit: oh yeah, can't use * for multiplication.




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