Maths Olympiad Prep

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Algebra Difficulty 5.5 AIME, harder Prove it United States

Problem:
Find the sum of all real numbers xx for which
x+x+x+x=2017 and {{{{x}+x}+x}}+x}=12017 \lfloor\lfloor\cdots\lfloor\lfloor\lfloor x\rfloor+x\rfloor+x\rfloor \cdots\rfloor+x\rfloor=2017 \text{ and }\{\cdots\{\{\{x\}+x\}+x\} \cdots\}+x\} = \frac{1}{2017}
where there are 20172017 xx's in both equations. (x\lfloor x\rfloor is the integer part of xx, and {x}\{x\} is the fractional part of xx.) Express your sum as a mixed number.

Solution

Solution:
The two equations are equivalent to 2017x=20172017\lfloor x\rfloor=2017 and {2017x}=12017\{2017 x\}=\frac{1}{2017}, respectively. The first equation reduces to x=1\lfloor x\rfloor=1, so we must have x=1+rx=1+r for some real rr satisfying 0r<10 \leq r<1. From the second equation, we deduce that {2017x}={2017+2017r}={2017r}=12017\{2017 x\}=\{2017+2017 r\}=\{2017 r\}=\frac{1}{2017}, so 2017r=n+120172017 r=n+\frac{1}{2017}, where nn is an integer. Dividing both sides of this equation by 20172017 yields r=n2017+120172r=\frac{n}{2017}+\frac{1}{2017^{2}}, where n=0,1,2,,2016n=0,1,2, \ldots, 2016 so that we have 0r<10 \leq r<1. Thus, we have x=1+r=1+n2017+120172x=1+r=1+\frac{n}{2017}+\frac{1}{2017^{2}} for n=0,1,2,,2016n=0,1,2, \ldots, 2016. The sum of these solutions is 20171+20162017212017+2017120172=2017+20162+12017=3025120172017 \cdot 1+\frac{2016 \cdot 2017}{2} \cdot \frac{1}{2017}+2017 \cdot \frac{1}{2017^{2}}=2017+\frac{2016}{2}+\frac{1}{2017}=3025 \frac{1}{2017}.

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