How many unordered pairs of positive integers satisfy the equation above?
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The solution of the problem is equivalent of finding the unordered pairs of (x, y) that satisfy the following equation: x 1 + y 1 = 2 0 0 6 3 ⇒ x y x + y = 2 0 0 6 3 ⇒ x + y 3 x y = 2 0 0 6 ⇒ 3 x y = 2 0 0 6 x + 2 0 0 6 y ⇒ 3 x y − 2 0 0 6 x = 3 2 0 0 6 ⋅ 3 y ⇒ x ⋅ ( 3 y − 2 0 0 6 ) = 3 2 0 0 6 ⋅ ( 3 y − 2 0 0 6 + 2 0 0 6 ) ⇒ x ⋅ ( 3 y − 2 0 0 6 ) = 3 2 0 0 6 ⋅ ( 3 y − 2 0 0 6 ) + 3 2 0 0 6 2 ⇒ ( x − 3 2 0 0 6 ) ⋅ ( 3 y − 2 0 0 6 ) = 3 2 0 0 6 2 ⇒ ( 3 x − 2 0 0 6 ) ⋅ ( 3 y − 2 0 0 6 ) = 2 0 0 6 2 ⇒ ( 3 x − 2 0 0 6 ) ⋅ ( 3 y − 2 0 0 6 ) = 2 2 ⋅ 1 7 2 ⋅ 5 9 2 By just observing the equation we see that 2 ≡ 1 7 ≡ 5 9 ≡ 2 ( m o d 3 ) Since x and y are symmetrical in this equation we are looking solutions to the equation 2 a ⋅ 1 7 b ⋅ 5 9 c + 2 0 0 6 ≡ 0 ( m o d 3 ) , 0 ≤ a , b , c ≤ 2 ⇒ 2 a ⋅ 2 b ⋅ 2 c + 2 ≡ 0 ( m o d 3 ) ⇒ 2 a + b + c + 2 ≡ 0 ( m o d 3 ) Thus: ( a + b + c ) = { 0 , 2 , 4 , 6 } But since we are looking for unordered pairs, we can limit ourselves such that: ( a + b + c ) = { 0 , 2 } − For (a + b + c) = 0 we get only 1 solution where ( a , b , c ) = ( 0 , 0 , 0 ) − For (a + b + c) = 2 we get 6 solutions where ( a , b , c ) = ( 1 , 1 , 0 ) , ( 1 , 0 , 1 ) , ( 0 , 1 , 1 ) , ( 2 , 0 , 0 ) , ( 0 , 2 , 0 ) , ( 0 , 0 , 2 ) Hence there are in total 7 solutions.