Pretty slick math trick.

Not a puzzle but it's neat anyway, a differant way to multiply:
http://www.shoutfile.com/v/fbpANixx/Easy_Mental_Multiplication_Trick

Originally posted by sonhouse
Not a puzzle but it's neat anyway, a differant way to multiply:
http://www.shoutfile.com/v/fbpANixx/Easy_Mental_Multiplication_Trick

Just hope that the numbers don't have lots of 9's.

BigDogg, has anyone ever told you that you're a bit of a downer? So many of your posts are condescending.

Originally posted by dmnelson84
BigDogg, has anyone ever told you that you're a bit of a downer? So many of your posts are condescending.

Yes, I'm skeptical by nature. Sorry, but the thought of that poor sap drawing nine lines for every 'nine', and then counting all the myriad intersections, and then doing all the 'carrying' just struck me as funny.

In your case, skeptical is just a euphemism for rude.

Originally posted by dmnelson84
In your case, skeptical is just a euphemism for rude.

I've got a pretty neat trick for doing multiplication in your head. Successive approximation.

To take the example in the video: 21 x 13.

10 * 2 * 13 = 260
+
1*13
=
273

And a harder example: 54 x 37

50 * 40 = 2000
-
50 * 3
1850
4 * 40 = 160
-
4 * 3
148
1850 + 148 = 1998

Why would you need more tricks than this?

Originally posted by dmnelson84
In your case, skeptical is just a euphemism for rude.

It appears I may have offended your delicate sensibilities somewhere, but I don't really care.

Thanks for proving my point.

Originally posted by dmnelson84
Thanks for proving my point.

By the way, what does all this have to with a "Math Trick" thread? Do you agree or disagree that the trick shown has little practical value?

Originally posted by dmnelson84
In your case, skeptical is just a euphemism for rude.

that's pretty rude

Neat trick, totally impractical. I just wonder how many people can prove why it works*.

😵


*should be trivial if you know how multiplication works (I too am a skeptic).

I disagree with most of the remarks made above. The principle is very good, and can be applied without doing the graphics (and regardless the number of 9's). Only, you have to do the exercise from right to left, so you can write down the solution from right to left character by character without overloading your memory. In real-life exercises, once the person has acquired the skill, multpilications of 5x5 numbers can be done 10+ times faster by the same person than with any other manual method.

Originally posted by zebano
Neat trick, totally impractical. I just wonder how many people can prove why it works*.

😵


*should be trivial if you know how multiplication works (I too am a skeptic).

Using marbles, I can represent 3*2 as 3 groups of two marbles each. The trick on the video simply substitutes line intersections for marbles. The lines are all drawn diagonally to keep the proper order of magnitude for each number.

However, try the intersection trick for 9*9, and you will see why we use times tables instead.