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Find the sum of all positive integers k k that can be represented in the form

x + 1 y + y + 1 x \dfrac {x+1}{y} + \dfrac {y+1}{x}

where x x and y y are also positive integers.


The answer is 7.

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2 solutions

Ankit Kumar Jain
Feb 19, 2017

The expression is equivalent to :

x 2 + y 2 + x + y x y \frac{x^2 + y^2 + x + y}{xy}

Since it is an integer ,

x y x 2 + y 2 + x + y xy | x^2 + y^2 + x + y

x y ( y + 1 ) , y x ( x + 1 ) \Rightarrow x | y(y + 1) , y | x(x + 1)

Now, since y , ( y + 1 ) y , (y + 1) are co-prime. Therefore , either x y x | y or x ( y + 1 ) x | (y + 1) and is co -prime to the other.

Therefore , one of the following holds x y , y x x | y , y | x or x y + 1 , y x + 1 x | y + 1 , y | x + 1 .

The former case leads to x = y x = y , using this in the expression forces x 2 ( x + 1 ) x = 1 , 2 x | 2(x + 1) \Rightarrow x = 1 , 2

Hence , two solutions is ( x , y ) = ( 1 , 1 ) ; ( 2 , 2 ) \boxed{(x , y) = (1 , 1) ; (2 , 2)}

The latter case x , y x + y + 1 \Rightarrow x , y | x + y + 1 .Since x , y x , y are co-prime , therefore x y x + y + 1 xy | x + y +1

x y x + y + 1 \Rightarrow xy \leq x + y + 1

( x 1 ) ( y 1 ) 2 \Rightarrow (x - 1)(y - 1) \leq 2

This leads to three pairs ( x , y ) = ( 2 , 3 ) ; ( 3 , 2 ) ; ( 2 , 2 ) \boxed{(x , y) = (2 , 3) ; (3 , 2) ; (2 , 2)} and hence we get four solutions namely, ( x , y ) = ( 1 , 1 ) ; ( 2 , 2 ) ; ( 2 , 3 ) ; ( 3 , 2 ) k = 3 , 4 \boxed{(x , y) = (1 ,1) ; (2 , 2) ; (2 , 3) ; (3 , 2)} \Rightarrow \boxed{k = 3 , 4}

When x>y or y>x is impossible, so x=y

2+(2/x) is integer so x=y=2 and x=y=1

The answer is 4+3=7

@Dipa Parameswara Although no additional values of k k are generated but your solution goes wrong because two other pairs ( 2 , 3 ) ; ( 3 , 2 ) (2 , 3) ; (3 , 2) also satisfy.

Ankit Kumar Jain - 4 years, 3 months ago

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Ohh i'm sorry

I Gede Arya Raditya Parameswara - 4 years, 3 months ago

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