Originally Posted by kcab
Originally Posted by Val
Have you tried teaching x! (x factorial, as opposed to Hurray for x!) ?
LOL, I was trying to figure out what Hurray for x! was, another math program perhaps, and what did it have to do with x factorial...

DS has long had an interest in triangular numbers - which I never knew had a name until just now. As it says over at Wikipedia, that's the additive analogue to factorial. When we play foosball (is that the spelling?) he insists on keeping score by summing the scores as well as the normal way. (As in, if I have 3 points it's also 1 + 2 + 3.) Comes up with other sequences too, like counting stairs. One of these days we'll have to investigate the pattern in more depth.

Sounds like a fun day, Kriston.

I used to have summer jobs that involved counting change in the register at the end of the day. I used to count the coins by putting them into pyramids with four coins at the bottom. Each pyramid had 10 coins, and so counting accurately became fast and easy. Other people noticed and starting making pyramids when it was their turn to count change!

I understand your son; I love number tricks like this.

Val