Author Topic: Fast multiplication with pen and paper (Read 6703 times)

Deep Toaster · « **on:** January 15, 2013, 08:10:17 pm »

Found a really neat trick on Lifehacker today. Might actually have been useful if I didn't have my calculator with me all the time.

I'll have to think a bit about how that works O.o

EDIT: I guess it's basically the same as multiplying it the normal way, except that in the step where you're adding numbers together you're adding columns of digits instead. Not as groundbreaking as I thought, but it's ... visual?

TheNlightenedOne · « **Reply #1 on:** January 15, 2013, 08:13:34 pm »

You group the 100's, 10's, 1's, etc. together, and then draw the other number perpendicular. So you multiply the 10's by the 10's, the 10's by the 1's, the 1's by the 1's, and the 1's by the 10's. I'm not explaining it well, but I understand how it works.

ruler501 · « **Reply #2 on:** January 15, 2013, 08:40:01 pm »

That is really interesting I might try using it one day in class. I always try to avoid using a calculator when I can.

Rhombicuboctahedron · « **Reply #3 on:** January 15, 2013, 10:20:23 pm »

This would really only be good with numbers like he showed, with 1, 2 and 3.
If you had 72*93, and you had to count 63 dots, or 95765412*841123, and you had to group and add a bunch of groups, it would become more difficult.
I guess I could just multiply 7*9=63 and then group them, but it seems like more work than just “carry the one”
Maybe if I was doing 1231213*3123311 or something, then it might be easier

Juju · « **Reply #4 on:** January 15, 2013, 10:28:28 pm »

I think I saw this one a long while ago, it's a pretty clever way to multiply. For the more complex ones (like 72*93), learning your multiplication tables is still pretty useful (7*9=63).

You guys should learn the multiplication tables from at least 1*1 to 10*10.

Sorunome · « **Reply #5 on:** January 15, 2013, 10:32:34 pm »

Wow, that is a pretty interesting trick, and, /why/ is it that way, lol?

Builderboy · « **Reply #6 on:** January 15, 2013, 11:04:26 pm »

This video talks about it nicely and goes over how it really isn't that much different as you might think! Vihart is also a great math channel for all sorts of other interesting things.

Sorunome · « **Reply #7 on:** January 15, 2013, 11:15:45 pm »

Ah, that makes a lot of sense!
Thanks for linking

aeTIos · « **Reply #8 on:** January 16, 2013, 04:37:40 am »

Like this if you know your multiplication tables!

* aeTIos hides

Author Topic: Fast multiplication with pen and paper (Read 6703 times)

Deep Toaster

Fast multiplication with pen and paper

TheNlightenedOne

Re: Fast multiplication with pen and paper

ruler501

Re: Fast multiplication with pen and paper

Rhombicuboctahedron

Re: Fast multiplication with pen and paper

Juju

Re: Fast multiplication with pen and paper

Sorunome

Re: Fast multiplication with pen and paper

Builderboy

Re: Fast multiplication with pen and paper

Sorunome

Re: Fast multiplication with pen and paper

aeTIos

Re: Fast multiplication with pen and paper