Determine all pairs ( x , y ) of integers satisfying the equation: 1 + 2 x + 2 2 x + 1 = y 2 If your answers are ( x 1 ; y 1 ) , ( x 2 ; y 2 ) , ⋅ ⋅ ⋅ , ( x n ; y n ) type it as y 1 x 1 + y 2 x 2 + ⋅ ⋅ ⋅ + y n x n
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not (4,+-31) but (4,+-23)
We notice that 1 + 2 x + 2 2 x + 1 is not an integer if x < − 1 and equals 2 and 4 when x = − 1 and 0 respectively.
Thus, the only solution is when x ≤ 0 is ( x , y ) = ( 0 , ± 2 ) .
If x ≥ 1 , then 1 + 2 x + 2 2 x + 1 is odd. Hence 8 ∣ ( y 2 − 1 ) = 2 x ( 2 x + 1 + 1 ) , so that x ≥ 3 .
From ( y − 1 ) ( y + 1 ) = 2 x ( 2 x + 1 + 1 ) and the g cd ( y − 1 , y + 1 ) = 2 since they are consecutive integers,
we get 2 y − 1 × 2 y + 1 = 2 x − 2 ( 2 x + 2 + 1 ) = k ( 8 k + 1 ) , where k = 2 x − 2 is a positive power of 2.
Since 2 y − 1 and 2 y + 1 are consecutive integers, there must exist positive odd integers r , s such that { 2 y − 1 , 2 y + 1 } = { k r , s } and r s = 8 k + 1 .
Since 1 = ∣ k r − s ∣ = ∣ k r − r 8 k + 1 ∣ , we must have r = 3 , Hence ∣ k − 1 ∣ = 3 and 2 x − 2 = k = 4 . Thus, x = 4 and y = ± 2 3 .
The only solutions in integer pairs are ( x , y ) are ( 0 , ± 2 ) and ( 4 , ± 2 3 ) ⟹ y 1 x 1 + y 2 x 2 + ⋅ ⋅ ⋅ + y n x n = 2 0 + ( − 2 ) 0 + 2 3 4 + ( − 2 3 ) 4 = 5 5 9 6 8 4 .
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