Find the number of positive integers n such that the following is also an integer:
n − 5 n 2 + 1 1 n + 2 .
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n − 5 n 2 + 1 1 n + 2 = n − 5 ( n + 1 6 ) ( n − 5 ) + 8 2 = n + 1 6 + n − 5 8 2
So we are looking for the number of integers n for which
n − 5 ∣ 8 2
which is just the number of positive divisors of 8 2 = 2 ⋅ 4 1 plus its negative divisors less than 5 , as 4 1 > 5 , this is just the factors of 2 and thus out answer is ( 1 + 1 ) ( 1 + 1 ) + 2 = 6
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Wow, I see my very silly error now, I am lucky to have gotten it correct by chance. I have fixed my solution accordingly
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We start by dividing the polynomial
n − 5 n 2 + 1 1 n + 2 = n + 1 6 + n − 5 8 2
Since we want this expression to be an positive integer so ( n − 5 ) ∣ 8 2
Then
n − 5 = ± 1 , ± 2 , ± 4 1 , ± 8 2
n = 4 , 6 , 3 , 7 , − 3 6 , 4 6 , − 7 7 , 8 7 but only 6 of these is positive ,so , the answer is 6