This is a little thing I put together, I haven't seen it anywhere on here so I figured I'd share....
In this method we use reverse logic by finding all the numbers that arn't prime and eliminating them. Since all of these primes (11-109) can only end in 1,3,7,or 9 we create a table for all possible outcomes. We notice a repeating pattern: (21, 51, 81), (33,63,93), (27,57,87), and (39,69,99) are all NOT prime and can be eliminated. This only leaves 49,77,and 91 as the other non-primes. Draw this matrix at the beginning of the test to help reference prime numbers. I've attached a better diagram if you have a hard time visualizing below. Hope this helps!
11 13 17 19
21 23 27 29
31 33 37 39
41 43 47 49
51 53 57 59
61 63 67 69
71 73 77 79
81 83 87 89
91 93 97 99
101 103 107 109
~Drew
In this method we use reverse logic by finding all the numbers that arn't prime and eliminating them. Since all of these primes (11-109) can only end in 1,3,7,or 9 we create a table for all possible outcomes. We notice a repeating pattern: (21, 51, 81), (33,63,93), (27,57,87), and (39,69,99) are all NOT prime and can be eliminated. This only leaves 49,77,and 91 as the other non-primes. Draw this matrix at the beginning of the test to help reference prime numbers. I've attached a better diagram if you have a hard time visualizing below. Hope this helps!
11 13 17 19
21 23 27 29
31 33 37 39
41 43 47 49
51 53 57 59
61 63 67 69
71 73 77 79
81 83 87 89
91 93 97 99
101 103 107 109
~Drew
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Last edited by scoowhoop on Tue Mar 09, 2010 12:15 am, edited 1 time in total.
