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Algebra Difficulty 4.8 AIME Find the answer

Let x<yx<y be positive real numbers such that x+y=4\sqrt{x}+\sqrt{y}=4 and x+2+y+2=5\sqrt{x+2}+\sqrt{y+2}=5. Compute xx.

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

Solution

Adding and subtracting both equations gives x+2+x+y+2+y=9x+2x+y+2y=1\begin{aligned} & \sqrt{x+2}+\sqrt{x}+\sqrt{y+2}+\sqrt{y}=9 \\ & \sqrt{x+2}-\sqrt{x}+\sqrt{y+2}-\sqrt{y}=1 \end{aligned} Substitute a=x+x+2a=\sqrt{x}+\sqrt{x+2} and b=y+y+2b=\sqrt{y}+\sqrt{y+2}. Then since (x+2+x)(x+2x)=2(\sqrt{x+2}+\sqrt{x})(\sqrt{x+2}-\sqrt{x})=2, we have a+b=92a+2b=1\begin{gathered} a+b=9 \\ \frac{2}{a}+\frac{2}{b}=1 \end{gathered} Dividing the first equation by the second one gives ab=18,a=3,b=6ab=18, a=3, b=6 Lastly, x=x+2+x(x+2x)2=3232=76\sqrt{x}=\frac{\sqrt{x+2}+\sqrt{x}-(\sqrt{x+2}-\sqrt{x})}{2}=\frac{3-\frac{2}{3}}{2}=\frac{7}{6}, so x=4936x=\frac{49}{36}.

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