Fraction of a fraction

1 + x 1 2 + x 1 2 + x 1 2 + \large 1+\frac { x-1 }{ 2+\frac { x-1 }{ 2+\frac { x-1 }{ 2+\ddots } } }

For integer 1 x 2015 1\le x\le 2015 , find the number of x x such that the infinitely nested fraction above is an integer.


The answer is 44.

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1 solution

Lee Isaac
Jun 16, 2015

Let the nested fraction =y

Then y = x 1 2 + y y=\frac{x-1}{2+y}

y 2 + 2 y = x 1 y^{2}+2y=x-1

y 2 + 2 y + 1 = x y^{2}+2y+1=x

( y + 1 ) 2 = x (y+1)^{2}=x

Since ( y + 1 ) 2 (y+1)^{2} is an integer, x must be a square number.

The question now is, how many natural numbers up to 2015 are square numbers?

2015 44.89 \sqrt {2015} \approx 44.89

Rounding down, we get the answer,

44 \boxed{44}

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