Hey guys, I just bought a book on mental calculation. It is called Think Like A Maths Genius, written by Arthur Benjamin and Michael Shermer. Some of the tricks are damn obvious, but I'll type it here anyway.
Chapter 0
Quick Tricks:
Easy (and impressive) Calculation
Instant Multiplication of 11
71 x 11
To solve this,, add the digits 7 + 1 = 8 in the middle of 7 and 1
71 x 11 = 781
How about
76 x 11
Similar way, 7 + 6 = 13
BUT the answer is not 7136
It should be done this way
1
736+
836
How about 3 digit numbers?
Simple. It can be done in similar way.
Eg. 123 x 11
1 + 2 = 3
2 + 3 = 5
123 x 11 = 1353
Multipying 2 2-digit number with the same first digit and second digit that sums to 10.
71 x 79
The first 2 digit will be 7 x 8 = 56
and The last 2 digit will be 1 x 9 = 9
so the answer is 5609.
*more will be added soon.
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