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Roots of Unity

How the Simpsons (And You) Can Multiply by Seven Using Your Fingers

3 times 7 is...25? What?

When I tutored elementary school math, one of my favorite things to show the kids was how to multiply by 9 using their fingers. (I apologize, but this trick assumes that you have five fingers on each hand. Later you’ll see how to adapt it if you have nonstandard hands.) Here’s how the finger rule [...]

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