c4countdown

http://www.youtube.com/watch?v=e-P5RGdjICo

This is an interesting little video of a technique taught in Japanese schools (allegedly, I've not really looked into it) for how to visually do large-number multiplications by drawing lines and counting the intersections.

Is this an extremely easy thing to explain in terms of why it works for you maths guys? Whether it is or it isn't I thought you might like to see it anyway.

Never seen this before, but I'll have a bash. For a 2 digit times by (heh heh) a 2 digit number:

ab x cd

is actually

(10a + b) x (10c + d)

which simplifies to 100ac + 10(ad + bc) + bd.

So you can see that the hundreds column comes from the number of times a and c would cross, the tens column comes from the number of times a crosses d PLUS b crosses c (carrying over to the hundreds column where necessary), and the units column from b crossing d (carrying over to the tens column where necessary).

It's quite sweet actually, but you need to be sure of getting your resulting columns right - if you have a 0 in your number remember that 'no intersections' needs to be counted too!

I've seen this before, is it called Vedic mathematics? If so, there is a lot more techniques along these lines which help with large multiplication and squaring numbers.