Find the sum of all prime numbers such that and are also primes.
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Let P be a prime so:
Case 1 : If P = 5
4 P 2 + 1 = 1 0 1 and 6 P 2 + 1 = 1 5 1 which are both primes so 5 is a solution
Case 2 : If P ≡ 1 , 2 , 3 , 4 ( m o d 5 )
So P 2 ≡ 1 , o r , − 1 ( m o d 5 )
if P 2 ≡ 1 ( m o d 5 ) so 4 P 2 + 1 ≡ 0 ( m o d 5 ) which is not prime.
if P 2 ≡ − 1 ( m o d 5 ) so 6 P 2 + 1 ≡ 0 ( m o d 5 ) which is not prime
So 5 is the only solution.