Inspired by Huan Le Quang

Find the number of positive integers n n such that the following is also an integer:

n 2 + 11 n + 2 n 5 . \frac{ n^2 + 11n + 2 } { n-5} .


Inspiration

3 4 5 6

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2 solutions

We start by dividing the polynomial

n 2 + 11 n + 2 n 5 = n + 16 + 82 n 5 \dfrac{n^{2}+11n+2}{n-5}=n+16+\dfrac{82}{n-5}

Since we want this expression to be an positive integer so ( n 5 ) 82 (n-5) | 82

Then

n 5 = ± 1 , ± 2 , ± 41 , ± 82 n-5=\pm 1,\pm 2, \pm 41, \pm 82

n = 4 , 6 , 3 , 7 , 36 , 46 , 77 , 87 n=4,6,3,7,-36,46,-77,87 but only 6 of these is positive ,so , the answer is 6 \boxed{6}

Sean Sullivan
Aug 6, 2015

n 2 + 11 n + 2 n 5 = ( n + 16 ) ( n 5 ) + 82 n 5 = n + 16 + 82 n 5 \frac{ n^2 + 11n + 2 } { n-5}=\frac{ (n+16)(n-5)+82 } { n-5}=n+16+\frac{82}{n-5}

So we are looking for the number of integers n n for which

n 5 82 n-5\mid82

which is just the number of positive divisors of 82 = 2 41 82=2\cdot41 plus its negative divisors less than 5 5 , as 41 > 5 41>5 , this is just the factors of 2 2 and thus out answer is ( 1 + 1 ) ( 1 + 1 ) + 2 = 6 (1+1)(1+1)+2=\boxed{6}

Are you sure?

Calvin Lin Staff - 5 years, 10 months ago

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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

Sean Sullivan - 5 years, 10 months ago

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Looks good now :)

Calvin Lin Staff - 5 years, 10 months ago

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