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Perfect Square Not (Posted on 2008-05-07) |
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The pairs of positive integers (A, B) are such that B divides 2A2.
Prove that A2 + B cannot be a perfect square.
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Submitted by K Sengupta
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Rating: 2.7500 (4 votes)
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Solution:
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At the outset, we substitute 2A2 = c*B. Since B divides 2A2, it follows that c must be a positive integer.
If possible, let us suppose that:
A2 + B = G2, for some positive integer G
Then, we have:
A2*c2 + B*c2 = G2*c2
or, A2*c2 + 2A2*c = G2*c2
or, A2(c2 + 2c) = (G*c)2......(*)
But, c2 < c2 + 2c < (c+1)2, and accordingly, the lhs of
(*) cannot be the square of a positive integer. This leads to a contradiction.
Consequently, A2 + B cannot be a perfect square.
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For an alternative methodology, refer to the solution submitted by Praneeth in this location.
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