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Algebra Difficulty 4.8 AIME Find the answer

Real numbers xx and yy satisfy the following equations: x=log10(10y1+1)1x=\log_{10}(10^{y-1}+1)-1 and y=log10(10x+1)1y=\log_{10}(10^{x}+1)-1. Compute 10xy10^{x-y}.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Taking 10 to the power of both sides in each equation, these equations become: 10x=(10y1+1)10110^{x}=\left(10^{y-1}+1\right) \cdot 10^{-1} and 10y=(10x+1)10110^{y}=\left(10^{x}+1\right) \cdot 10^{-1}. Let a=10xa=10^{x} and b=10yb=10^{y}. Our equations become: 10a=b/10+110a=b/10+1 and 10b=a+110b=a+1 and we are asked to compute a/ba/b. Subtracting the equations gives 10a10b=b/10a11a=101b/1010a-10b=b/10-a \Longrightarrow 11a=101b/10 giving an answer of 101/110101/110.

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