a·(b+c) = a·b + a·c ?

It’s called the distributive law of equality.

You can use it to make hard math questions easier.

For example:

19·20 = ?

Well, I know that 19·20 = (20-1)·20

And I know that 20·20 = 400

So 19·20 = 20·20 – 1·20 = 380

I say out loud, “nineteen twenties equals twenty twenties minus one twenty – which equals four hundred minus twenty – which equals three hundred and eighty.”

I can do that in my head!

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