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Floored parabola (Posted on 2023-04-03) Difficulty: 2 of 5
How many real solutions does the equation ⌊x2⌋ = (x - 150)2 have? How many are rational?

No Solution Yet Submitted by Jer    
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Hints/Tips re: I think ... | Comment 3 of 4 |
(In reply to I think ... by Larry)

Looking at positive x and its associated integer N: For any interval [sqrt(N), sqrt(N+1)) the left side is constant but the right side is continuously increasing so there can only be at most one possible solution.


So lets take your found root x=sqrt(N)+1/50.  This will be the intersection, so long as it is still within the interval.  Then we need sqrt(N)+1/50 < sqrt(N+1).  Square each side and simplify and we get sqrt(N) < 24.99, or N < 624.5001.

So then there are 625 integer values of N that work according to your formula: N=0 to 624.  Charlie's brute force search matches this range.

  Posted by Brian Smith on 2023-04-03 11:44:13
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