Math Is Fun Forum

Haiying Liao

Guest

finger tricks about multiplication table ???

Fingers are a set of manipulative that most people have, and they can be used to help students remember their multiplication facts for 9. This is how the method works. Suppose you want to know 9 x 7. Bend down finger number 7. The number of fingers to the left of the bent finger gives you the value of the tens place of the product, and the number of fingers to the right of the bent finger gives you the value of the ones place of the product.

WHY DOES THIS WORK?

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: finger tricks about multiplication table ???

hi Haiying Liao

It's a great trick, isn't it?

It is a property of the nine times table (up to 10 x 9) that the tens and units digits add up to 9 and that if these digits add up to 9, then the number is in the table.

So putting down one finger creates two numbers (left of finger; right of finger) that have the property and so are in the table.

Furthermore n x 9 has a tens digit of (n-1) so the finger down gives the correct value in the table.

It remains to prove the property.

Post back if you want me to show you that.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

gr8girl101

Member

Registered: 2011-02-03

Posts: 6

Re: finger tricks about multiplication table ???

That is so interesting! I never knew how it worked before.

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"Never Never Never give up!"
-Winston Churchill
~~~~~~~~~♥☺☮☀~~~~~~~♡✌❖✬~~~~~~~~~~GO NY JETS!!!~~~~~~~

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: finger tricks about multiplication table ???

hi gr8girl101

You are welcome.  Love the avatar! 

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob