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.
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!