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Algebra Difficulty 4.8 AIME Find the answer

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}

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

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=12 \sqrt{x+2}-2 \sqrt{x-2}=1 Squaring, we get that 8x8(x+2)(x2)=18x1=8(x+2)(x2)8 x-8 \sqrt{(x+2)(x-2)}=1 \Rightarrow 8 x-1=8 \sqrt{(x+2)(x-2)} Squaring again gives 64x216x+1=64x225664 x^{2}-16 x+1=64 x^{2}-256 so we get that x=25716x=\frac{257}{16}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.