Stage 3 · Structure / #15
Why Is a Negative Times a Negative Positive?
Coming soon
The video is in production. Follow along on GitHub to hear when it is out.
The idea
Think of a number as an arrow from zero: multiplying by a positive number stretches it, and multiplying by negative one turns it half a turn around zero. Negative two times negative three is stretch to six, then two half turns, back to positive six.
A common mix-up
Negative times negative being positive has no reason, you just memorise it; two minus signs cancel, so negative two plus negative three is positive five.
Where it stops working
Adding has no such rule: negative two plus negative three starts at negative two and goes three more to the left, to negative five.
School year
China 7 US 6