ab * ac where ab & ac are two-digit numbers, and b+c=10.
In reality, (10a+b)(10a+c) = 100a^2+10ac+10ab+bc
Some reverse distributing, 10a(10a+c+b)+bc
Since b+c=10 and b+c=c+b, then 10a(10a+c+b)+bc = 10a(10a+10)+bc = 100a(a+1)+bc.
This trick is only a corollary of that general rule the gentleman instructed upon.
it works for any double digit numbers, with the 1st digits being the same and the 2nd digits summing up to 10.
ie. 43×47, just multiply 1st digit by next higher (in this case 5, so 4×5=20), and write the product of the last 2 digits (in this case 3×7=21)
and there you have it,
43×47=2021
25 Comments
December 11th, 2009 at 7:58 am
That will come in very useful in my Physics class.
December 11th, 2009 at 8:19 am
yea..if u read the info and listened he said that it’s only for numbers that end in 5
December 11th, 2009 at 8:59 am
but the square of 12 is 144 while in ur way is 24 is that onnly if the last number is 5??? nice try!!”=)
December 11th, 2009 at 9:54 am
nice vid, the sound quality wasn’t the greatest though
December 11th, 2009 at 10:11 am
does 777 edd with 5?
December 11th, 2009 at 11:08 am
ok i take that back..777×777 dosent work…..why dont you get high off that dry erase marker you virgin for life
December 11th, 2009 at 11:27 am
wow thats awsome
December 11th, 2009 at 12:19 pm
Hey, thanks! I’d never seen it proven before.
December 11th, 2009 at 12:52 pm
Proof for this trick…
ab * ac where ab & ac are two-digit numbers, and b+c=10.
In reality, (10a+b)(10a+c) = 100a^2+10ac+10ab+bc
Some reverse distributing, 10a(10a+c+b)+bc
Since b+c=10 and b+c=c+b, then 10a(10a+c+b)+bc = 10a(10a+10)+bc = 100a(a+1)+bc.
This trick is only a corollary of that general rule the gentleman instructed upon.
December 11th, 2009 at 1:32 pm
it is only with the 5 at the end? i tried 72*72
December 11th, 2009 at 2:06 pm
it works for any double digit numbers, with the 1st digits being the same and the 2nd digits summing up to 10.
ie. 43×47, just multiply 1st digit by next higher (in this case 5, so 4×5=20), and write the product of the last 2 digits (in this case 3×7=21)
and there you have it,
43×47=2021
December 11th, 2009 at 2:48 pm
hmmm… not very interesting… much better the hot chicks from LikeHerAss.com_
December 11th, 2009 at 3:14 pm
see how a hot chick loses a bet and has to strip in front of her classmates (_stripAss.com_)
December 11th, 2009 at 4:04 pm
I see I’m not the only one who thinks so. Wish I could write with cool flourishes like that.
Oh, nice math trick, too.
)
December 11th, 2009 at 4:38 pm
well, does 777 end in 5?
December 11th, 2009 at 5:35 pm
Awesome!!!
December 11th, 2009 at 6:33 pm
fuck off u twat, i knew this years ago
it indian vedic math, stop stealing our stuff u honkey
December 11th, 2009 at 6:35 pm
Try 777×777 it does not work guys!
December 11th, 2009 at 7:03 pm
Ummm this only works with the last digit being 5
December 11th, 2009 at 7:47 pm
im going to tell my math mate about that, he’ll love it
December 11th, 2009 at 8:04 pm
nice. i’ll use that for the number sense test
December 11th, 2009 at 8:21 pm
Wo….your hand writing is amazing!
December 11th, 2009 at 8:23 pm
Wow… Cool..I love math tricks.. Hey, I like your handwriting style. Mine is like chicken scratches!
December 11th, 2009 at 8:26 pm
That was a good one.
December 11th, 2009 at 8:52 pm
Nice, Have you ever heard of the rule of 72? That one can make you wealthy.