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GDB Says, in 12-29-2007 at 12:43:59     

Wow, I am impressed!

Nick Says, in 12-29-2007 at 13:36:06     

Yeah it is very cool :)

Kenny Says, in 12-29-2007 at 19:11:09     

Certain educated religious groups would stone anyone who used that method on the grounds that it is unnatural.

Steven Smit Says, in 12-30-2007 at 00:58:38     

That was awesome. I’ve also been trying to find ways to solve to the power of sums like this.

Toon Says, in 12-30-2007 at 02:40:01     

It’s a cool way to solve certain problems yeah, but you can only use it to multiply certain numbers… If one of the numbers you want to multiply is too low, you can’t solve it. Let’s say you want to multiply 57 with 43. In this case you’d get something like this:

57 43 | 20
| | or
63 37 | 20

In this case, you’d still have to multiply 43 with 37… And again, you’re stuck with a hard multiplication… Or is there something I’m missing?

somejackass Says, in 12-31-2007 at 18:40:50     

Toon, you’re not missing anything. It only works for numbers very close to 100 (although I suppose you could do the same trick with other numbers).

When I saw the problem, I just intuitively said, ok, 98=100-2, and solved 100*87 – 2*87, which isn’t significantly harder in this case, and generalizes better.

BootstrapsBootstraps Says, in 1-1-2008 at 01:33:15     

I agree with you, Toon. In that example you mentioned the method doesn’t work at all. Multiplying 43 with 37 gives 1591, which would give an overall answer of 201591. The actual answer is 3591.

Matt` Says, in 1-3-2008 at 01:25:18     

There was a strange method I saw ages ago and can’t remember the details of, where you could work out multiplications involving 7s 8s and 9s using your fingers.

Since every digit in the example is 7 or greater, it’s probably related, but taken up an order of magnitude (the thing I saw was probably based on the same idea, but using 10 in the place of 100)

Jackolantern Says, in 1-6-2008 at 09:42:37     

Give credit where it’s due. I’m pretty sure this is in ancient Hindu texts. It’s called Vedic Math, and this is simply one principle. There are tons more for different kinds of problems, involving different numbers, etc. There are even classes for this stuff, but it’s pretty rare. Amazing stuff really, thank the Hindus.

asdf Says, in 1-7-2008 at 00:12:18     

Bootstraps, the answer does work if you add 2000 to 1591. The “sticking a number before another number” method only works if the numbers, when subtracted from 100, multiply to 100 or less.

turk Says, in 1-7-2008 at 05:31:09     

Interesting but useless. Old fashion way is faster. Faster way of multiplication is always the best way.

me Says, in 1-10-2008 at 16:58:15     

“I agree with you, Toon. In that example you mentioned the method doesn???t work at all. Multiplying 43 with 37 gives 1591, which would give an overall answer of 201591. The actual answer is 3591.”

actually yes it would work if you do the last step a little differently. anything over the tens place in your product should be added… 1591 would be 15+20=35 then add your tens and ones to the end of it….3591

UncleCharlie63 Says, in 1-24-2008 at 09:37:20     

Try 22 X 78. It doesn’t work.

Fine maths « Springblog08’s Weblog Says, in 3-4-2008 at 23:00:02     

[…] 4, 2008 at 9:00 pm · Filed under learning Did you know this way to multiply […]

word Says, in 3-13-2008 at 06:16:54     

math sucks

Yeah Says, in 4-13-2008 at 03:35:14     

This is based off multiplying two numbers of the form (100 – x). And so as long as you understand the method used, it will work for any number.

Let Z = 100 – y and W = 100 – x, then

Z*W = (100 – x) * (100 – y) = (100 – x – y)*100 + x*y

Yeah Says, in 4-13-2008 at 03:44:00     

Oh yeah, it also works for numbers larger than 100.

101 * 102 = (100 – (-1) – (-2))*100 + (-1)*(-2)
= 10300 + 2 = 10302

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