The numbers 3^n -1 are never prime (if n 2>= 2), since they are always even. However,
it sometimes happens that (3^n -1) /2 is prime. For example, (3^3 -1) /2 = 13 is prime.
a) If n is even, show that (3^n - 1)/2 is always divisible by 4, so it can never be prime.
b)Use a similar argument to show that if n is a multiple of 5 then (3^n - 1)/2 is never a prime.