## Re: [Mental Calculation] prime factors

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• By using Microsoft Excel, I found that:- 10001 = 73 x 137 9999 = 101 x 99   Therefore followed what had been discussed by Andie on 1001 & 999, we can use
Message 1 of 7 , Jul 31, 2008
By using Microsoft Excel, I found that:-
10001 = 73 x 137
9999 = 101 x 99

Therefore followed what had been discussed by Andie on 1001 & 999, we can use alternate addition and subtraction of four digits group to determine the prime factors of 73 and 137. (only when else fail and you are stucked) We can also use addition of four digits group to determine prime factor of 101 and factor of 99. Of course we can use alternate addition and subtraction of two digits to determine prime factor of 101.

For 10001, e.g. for number 197389737, 9737-9738 + 1 = 0, therefore 197389737 is evenly divisible by 73 and 137.
For 9999, e.g. for number 129137085, 7085+2913+1= 9999, therefore 129137085 is evenly divisible by 101 and 99 (9 x 11).
For 101, e.g. for number 9336642, 42-66+33-9=0, therefore 9336642 is evenly divisible by 101.

Just my thought on the subject.
Regards
Frank Chin

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