Sweepkick … you are real ambitious man… lol
True, but there is a difference between the two. Adding the digits for multiples of nine until you have a single digit always reduces to exactly nine. Multiples of three can reduce to 3, 6, or 9.
The divisibility formula that’s really freaky is divisibility by 7. Take a multidigit number. Delete the last digit, double it, then subtract the result from the remaining digits. If the result is evenly divisible by 7 , the original number was also. This method sometimes reduces to 0, -7, or -14, but those are evenly divisible by seven, so the rule holds.
For example: 4333. Take off the final 3, double it, and subtract that from the remaining digits: 433-6=427. Repeating, take off the seven, double it, and subtract: 42-14=28. You now have a multple of 7, so the original 4333 is a multiple.
wow. you are a mathmatician Number Six ??
I taught middle school pre-algebra for two years (along with seventh and eighth grade English), but no, I’m not a mathemetician. I actually learned that trick for divisibility by 7 in 5th grade, and it’s stuck with me ever since, just 'cause it seems so cool.
The rule of nines is just a particular case as in base n it will work with (n-1).
That rule of divisibility by seven can be proved by proving
Mod7(10n+m) = Mod7(n-2m)
Unfortunately I forgot what I knew of modular math. Can someone do it?
it is really so cool for a rule and I am really impressed you still remember that
Teaching is one of the best ways of learning something, is not it ???
Teaching something to another is one of the best ways of reinforcing something you’ve learned, and reinforcing knowledge is one of the steps in learning, so yes.
well, well said
that is actually what I meant to say, but of course, you have the right expressions…
all the difference bt a mother and a non-mother tongue speakers …