Math aka Fun Equations

[poorpete]

My daughter is learning multiplication and is learning a bunch of strategies, some of which I've never known. So I think my brain was already in this mode when I came up with a fun equation, dunno if this is common sense or well known, but it's fun.

n*n - y*y = (n+y)*(n-y)
aka
6*6 -1 = 7*5
100*100 - 4 = 98*102
25*25 - 9 = 22*28

or like 17*3, its average is 10, their distance from the average is 7. so it's 100-49.


Got any favorite equations?

Or, ever figure out something with your brain that you find out later is common knowledge?