Mathematics Magazine for Grades 1-12  

4/2004

 

     

Math Tricks

Finger Math: 9X Rule
To multiply by 9,try this:
(1) Spread your two hands out and place them on a desk or table in front of you.
(2) To multiply by 3, fold down the 3rd finger from the left. To multiply by 4, it would be the 4th finger and so on.
(3) the answer is 27 ... READ it from the two fingers on the left of the folded down finger and the 7 fingers on the right of it.
This works for anything up to 9x10!
Square a 2 Digit Number Ending in 5
For this example we will use 25 Take the "tens" part of the number (the 2 and add 1)=3
Multiply the original "tens" part of the number by the new number (2x3)
Take the result (2 x 3= 6) and put 25 behind it. Result the answer 625.
Try a few more 75 squared ... = 7 x 8 = 56 ... put 25 behind it is 5625.
55 squared = 5 x 6=30 ... put 25 behind it ... is 3025. Another easy one!
Practice it on paper first!

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
With practice this can easily be done in your head.