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Old April 18th, 2008, 01:34 PM
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Default Mental calculations tips..

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 !
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Last edited by Moo; August 21st, 2008 at 09:02 AM. Reason: calculation, not calculus...
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