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
November 13th, 2009 at 3:33 pm
That will come in very useful in my Physics class.
November 13th, 2009 at 4:31 pm
yea..if u read the info and listened he said that it’s only for numbers that end in 5
November 13th, 2009 at 4:54 pm
but the square of 12 is 144 while in ur way is 24 is that onnly if the last number is 5??? nice try!!”=)
November 13th, 2009 at 5:20 pm
nice vid, the sound quality wasn’t the greatest though
November 13th, 2009 at 5:34 pm
does 777 edd with 5?
November 13th, 2009 at 6:07 pm
ok i take that back..777×777 dosent work…..why dont you get high off that dry erase marker you virgin for life
November 13th, 2009 at 6:12 pm
wow thats awsome
November 13th, 2009 at 6:30 pm
Hey, thanks! I’d never seen it proven before.
November 13th, 2009 at 7:28 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.
November 13th, 2009 at 7:36 pm
it is only with the 5 at the end? i tried 72*72
November 13th, 2009 at 8:16 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
November 13th, 2009 at 8:21 pm
hmmm… not very interesting… much better the hot chicks from LikeHerAss.com_
November 13th, 2009 at 8:51 pm
see how a hot chick loses a bet and has to strip in front of her classmates (_stripAss.com_)
November 13th, 2009 at 8:58 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.
)
November 13th, 2009 at 9:29 pm
well, does 777 end in 5?
November 13th, 2009 at 9:51 pm
Awesome!!!
November 13th, 2009 at 10:39 pm
fuck off u twat, i knew this years ago
it indian vedic math, stop stealing our stuff u honkey
November 13th, 2009 at 11:18 pm
Try 777×777 it does not work guys!
November 13th, 2009 at 11:45 pm
Ummm this only works with the last digit being 5
November 13th, 2009 at 11:46 pm
im going to tell my math mate about that, he’ll love it
November 14th, 2009 at 12:23 am
nice. i’ll use that for the number sense test
November 14th, 2009 at 12:50 am
Wo….your hand writing is amazing!
November 14th, 2009 at 12:59 am
Wow… Cool..I love math tricks.. Hey, I like your handwriting style. Mine is like chicken scratches!
November 14th, 2009 at 1:30 am
That was a good one.
November 14th, 2009 at 1:44 am
Nice, Have you ever heard of the rule of 72? That one can make you wealthy.