Around four years ago, I saw an interesting post by Prof. Vincenzo Giordano about numerical curiosities. It showed how we can quickly multiply two equal numbers containing only 9s. Such as 9*9, 99*99, 999*999 and so forth. The picture below shows some examples with a general rule on how to get to the result of the multiplication.
Source post: https://www.facebook.com/permalink.php?story_fbid=3084055415031362&id=430003697103227
Being passionate about numbers, the post quickly triggered my curiosity, and I started thinking on whether we can create a more general rule that allows us to quickly multiply any combinations of 9-digits numbers, i.e. when multiplier and multiplicand have different quantity of 9s. For example 9*99, 9*999, 9*9999, 99*999 and so on.
I reverse-engineered these multiplications, and it turns out it is indeed possible to derive such a general rule.
Here you can find the rule:
This post was originally published on Facebook, a few days after Prof. Giordano’s one. My FB post in 2021: https://www.facebook.com/share/p/1A8SnQroZY/. Only now I have decided to report it here too.
