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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.
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