Find the second largest two-digit number such that does not divide
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I claim that n is a prime number. So , suppose n is not a prime number. This means we can express n = p × q for some positive integers p , q such that 1 < p , q < n . Let S be a set such that S = { 1 , 2 , 3 , 4 … , p , … , q , … , ( n − 1 ) } . Note that ( n − 1 ) ! is the product of all elements of S .Thus, p ∣ ( n − 1 ) ! , q ∣ ( n − 1 ) ! which means p q ∣ ( n − 1 ) ! ⇒ n ∣ ( n − 1 ) ! . So n must be prime so as to not divide ( n − 1 ) ! , thus the second largest two digit prime number is 8 9 .
As pointed by Svatejas S , the answer is trivial by Wilson's Theorem.