If there is such a thing as a 'cool' math trick, this would be it

biko

Just had to share this one

FuzzyWuzzyWazza

biko wrote:

Just had to share this one

seen this years ago (not the actual video) think it was when I was in primary school, it's pretty cool.

vallo

Cooool. Can't wait to show this off.

Misticles

cool.... will go down a treat!

mathias

Cool , but who knows why it works , post answers please , If anyone is interested I'll post it if no one else figures it out !!

bbbbb

21 x 13 is the same as
(20 + 1) x (10 + 3) is the same as
(2x10 + 1) (1x10 + 3) is the same as
2x10 x 1x10 + 2x10 x 3 plus
1 x 1x10 + 1x3

you can draw this graphically, e.g. 2 lines one direction crossing 3 lines perpindicular will intersect 6 times (2x3 !!!)
you'll have clusters of points crossing for the 1's 10's 100's etc.
It's only elegant for the lower no.s otherwise there's too much counting and carrying over to be done.

mathias

Yup , thats it , its a graphical representation of tens places ,

Just another little question , can anyone think of case where it wont work !! At least not in its presented form anyway , ( there are lots ! )

breadmonkey

I'll guess that it won't work for

276i*cos(sinh-1(9081273512735))

mathias

I was thinking more along the lines of 10 x 10 , 100 x 100 , 1000 x 1000 etc , all these should give the same answer using that system , which would be 1 !!!

As far as I can see , if there is a 0 in any of the numbers being multiplied , you run into problems !