
#1
Nov2106, 07:54 PM

P: 1

I've got a very helpfull system for making calculations, more exactly: multiplying.
How do you calculate large multiplications like e.g. 1538x483 Takes some time, isn't it? Well, I found this system for solving the problem and it's great. Whatch this, amazing: http://www.metacafe.com/watch/308408...iplying.%20Don Greetz Domino Edit: I see it has been removed today... strange. 



#2
Nov2106, 08:33 PM

Mentor
P: 7,292

What I see is just a complexification of normal multiplication. Whats the point?




#3
Nov2106, 09:01 PM

P: 223

I'm sorry, but this is pretty stupid ... This is merely obfuscation of the normal polynomial time, gradeschool multiplication.
If you want some "amazing" systems for mulitplying large numbers, look up the divideandconquer selection algorithm, or Fast Fourier Transforms. Or, for a system that is doable by hand use the system Al Khwarizmi discovered, one that is used today in some European countries. It works by synthesizing a binarystyled multiplication. You multiply and divide number a and number b, respectively, by 2, then strike out the even rows and add up column b. I won't bother showing how it works, because this is utterly pointless. The point being, that the technique outlined in that video is nothing but a rewrite of the gradeschool algorithm, and nowhere near "the best way to multiply large numbers". 



#4
Nov2406, 08:44 AM

HW Helper
P: 1,007

The best way to multiply!
I like to use the FOIL method for multiplication of 2 or 3 digit numbers, e.g.
79x91=(801)(90+1)=7200+80901=7189 which is not very fast, but I don't care to memorize much of my times table, so it helps me. 



#5
Nov2406, 09:08 AM

Emeritus
Sci Advisor
PF Gold
P: 11,154

[tex]79\times 91=(856)(85+6)=85^26^2=722536=7189[/tex] Different strokes! 



#6
Nov2406, 04:16 PM

P: 10

Hi  I didn't see the original (removed) link, but suggest you check out a famous book called Figuring by Shakuntala Devi. All the above methods are just basic examples of the Vedic system she resuscitated. Older members may remember she's the prodigy whose mental arithmetic beat the calculator on Blue Peter!
My favourite is squaring twodigit numbers ending in 5: eg 75 squared. You just take the first number, 7 in this case, and add 1, getting 8; multiply these together, giving 56; stick 25 on the end to give 5625, the answer. Amaze your friends! The general methods allow multiplication of long numbers with the answer on the line below! eg (deliberately simple example) 3 1 2 1 2 1 37752 Working from left to right: (3x1) 3 (3x2 + 1x1) 7 (3x1 + 1x2 + 2x1) 7 (1x1 + 2x2) 5 (2x1) 2 Check it out, it's seriously quick! Best wishes x=7 


Register to reply 
Related Discussions  
Is it possible to multiply by 'undefined'?  Linear & Abstract Algebra  7  
How to do 1565 x 1197 in 10 seconds without a calculator and paper?  Brain Teasers  13  
How to multiply two groups  General Math  2  
how to multiply two matrices??  Linear & Abstract Algebra  11  
can i just multiply these?  Introductory Physics Homework  2 