Find the sum of all positive integers that can be represented in the form
where and are also positive integers.
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The expression is equivalent to :
x y x 2 + y 2 + x + y
Since it is an integer ,
x y ∣ x 2 + y 2 + x + y
⇒ x ∣ y ( y + 1 ) , y ∣ x ( x + 1 )
Now, since y , ( y + 1 ) are co-prime. Therefore , either x ∣ y or x ∣ ( y + 1 ) and is co -prime to the other.
Therefore , one of the following holds x ∣ y , y ∣ x or x ∣ y + 1 , y ∣ x + 1 .
The former case leads to x = y , using this in the expression forces x ∣ 2 ( x + 1 ) ⇒ x = 1 , 2
Hence , two solutions is ( x , y ) = ( 1 , 1 ) ; ( 2 , 2 )
The latter case ⇒ x , y ∣ x + y + 1 .Since x , y are co-prime , therefore x y ∣ x + y + 1
⇒ x y ≤ x + y + 1
⇒ ( x − 1 ) ( y − 1 ) ≤ 2
This leads to three pairs ( 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