Twin prime
Number Theory
Level
3
Prove that when you multiply a pair of twin primes you get a number that has remainder 8 after division by 9. With one exception. [https://en.wikipedia.org/wiki/Twin_prime) tell that exception.
3, 5
43, 71
5, 7
7, 17
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1 solution
Any twin prime greater than 3 can be written as 3n-1,3n+1. so 3n-1*3n+1=9n^2-1. now it is straight.