Maths Olympiad Prep

Library / /37 of 82

Algebra Difficulty 5.0 AIME Prove it United States

Problem:
Find all real values of xx for which
1x+x2+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+x2+1x+2+x=xx2(x+x2)(xx2)+x+2x(x+2+x)(x+2x)=xx22+x+2x2=12(x+2x2), \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+22x2=1 2 \sqrt{x+2} - 2 \sqrt{x-2} = 1
Squaring, we get that
8x8(x+2)(x2)=18x1=8(x+2)(x2). 8x - 8\sqrt{(x+2)(x-2)} = 1 \Rightarrow 8x - 1 = 8\sqrt{(x+2)(x-2)}.
Squaring again gives
64x216x+1=64x2256 64x^{2} - 16x + 1 = 64x^{2} - 256
so we get that x=25716x = \frac{257}{16}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.