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Saturday, June 10, 2006

Mental Math Tricks

Tricks! What's the difference between a strategy and a trick? A trick is associated with magic, something happening without an obvious explanation. Mental arithmetic tricks are just the same. When you first learn them you may not know why they work but they are very useful nevertheless. They are even more useful when you are able to explain why they work and understand their limitations.
Here are some mathematical tricks for you to try ...
Multiply Up to 20X20 In Your Head
With this trick, you will be able to multiply any two numbers from 11 to 19 in your head quickly, without the use of a calculator. I will assume that you know your multiplication table reasonably well up to 10x10.
Now try this:
· Take 15 x 13 for an example.
· Always place the larger number of the two on top in your mind.
· Then draw the shape of Africa mentally so it covers the 15 and the 3 from the 13 below. Those covered numbers are all you need.
· First add 15 + 3 = 18
· Add a zero behind it (multiply by 10) to get 180.
· Multiply the covered lower 3 x the single digit above it the "5" (3x5= 15)
· Add 180 + 15 = 195
That is It! Wasn't that easy? Practice it on paper first!
Here's another one:
Square 2 Digit Number: UP-DOWN Method
Square a 2 Digit Number, for this example 37

-Look for the nearest 10 boundary
-In this case up 3 from 37 to 40.
-Since you went UP 3 to 40 go DOWN 3 from 37 to 34.
-Now mentally multiply 34x40
-The way I do it is 34x10=340;
-Double it mentally to 680
-Double it again mentally to 1360
-This 1360 is the FIRST interim answer.
-37 is "3" away from the 10 boundary 40.
-Square this "3" distance from 10 boundary.
-3x3=9 which is the SECOND interim answer.
-Add the two interim answers to get the final answer.
-Answer: 1360 + 9 = 1369

Confused? Practice.... practice... ^^


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