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Harvard-MIT November Tournament

Algebra Difficulty 5.0 AIME Prove it United States

Problem:
Find all real values of xx for which
1x+x−2+1x+2+x=14 \frac{1}{\sqrt{x}+\sqrt{x-2}}+\frac{1}{\sqrt{x+2}+\sqrt{x}}=\frac{1}{4}

Solution

Solution:
We note that
14=1x+x−2+1x+2+x=x−x−2(x+x−2)(x−x−2)+x+2−x(x+2+x)(x+2−x)=x−x−22+x+2−x2=12(x+2−x−2), \begin{aligned} \frac{1}{4} &= \frac{1}{\sqrt{x}+\sqrt{x-2}}+\frac{1}{\sqrt{x+2}+\sqrt{x}} \\ &= \frac{\sqrt{x}-\sqrt{x-2}}{(\sqrt{x}+\sqrt{x-2})(\sqrt{x}-\sqrt{x-2})} + \frac{\sqrt{x+2}-\sqrt{x}}{(\sqrt{x+2}+\sqrt{x})(\sqrt{x+2}-\sqrt{x})} \\ &= \frac{\sqrt{x}-\sqrt{x-2}}{2} + \frac{\sqrt{x+2}-\sqrt{x}}{2} \\ &= \frac{1}{2}(\sqrt{x+2}-\sqrt{x-2}), \end{aligned}
so that
2x+2−2x−2=1 2 \sqrt{x+2} - 2 \sqrt{x-2} = 1
Squaring, we get that
8x−8(x+2)(x−2)=1⇒8x−1=8(x+2)(x−2). 8x - 8\sqrt{(x+2)(x-2)} = 1 \Rightarrow 8x - 1 = 8\sqrt{(x+2)(x-2)}.
Squaring again gives
64x2−16x+1=64x2−256 64x^{2} - 16x + 1 = 64x^{2} - 256
so we get that x=25716x = \frac{257}{16}.

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