Math Help Forum - View Single Post

ice_syncer · #12 April 20th, 2008, 08:43 AM

Quote:

Originally Posted by Moo

Hello,

1.*1.

This has nothing to do with extraordinary technique, but you gotta think about it (I'm confident that some of you know about it)..

$(10+a)(10+b)=10(10+a)+b(10+a)=10(10+a)+10b+ab=10(10+a+b)+ab$

So multiplying two such numbers is equivalent to :

- sum one of the numbers with the unit digit of the other one
- multiply it by 10
- add the product of the two unit digits.

$19 \times 17= ?$

- $17+9=26$

- $26 \times 10=260$

- $7 \times 9=63$

- $19 \times 17=260+63=323$

I hope this would help at least one person in the future

Enjoy calculations !

well, pretty good, but I knew the mental multiplication tricks, all you have to do is dismantle the multiplication steps and add