Maths Olympiad Prep

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Algebra Difficulty 5.8 AIME, harder Prove it United States

Problem:
Let
A=limni=02016(1)i(ni)(ni+2)(ni+1)2 A = \lim_{n \rightarrow \infty} \sum_{i=0}^{2016} (-1)^i \cdot \frac{\binom{n}{i} \binom{n}{i+2}}{\binom{n}{i+1}^2}
Find the largest integer less than or equal to 1A\frac{1}{A}.
The following decimal approximation might be useful: 0.6931<ln(2)<0.69320.6931 < \ln(2) < 0.6932, where ln\ln denotes the natural logarithm function.

Solution

Solution:
Note
i=02016(1)i(ni)(ni+2)(ni+1)2=i=02016(1)i(i+1)(ni1)(i+2)(ni) \sum_{i=0}^{2016} (-1)^i \cdot \frac{\binom{n}{i} \binom{n}{i+2}}{\binom{n}{i+1}^2} = \sum_{i=0}^{2016} (-1)^i \cdot \frac{(i+1)(n-i-1)}{(i+2)(n-i)}
So
limni=02016(1)i(ni)(ni+2)(ni+1)2=i=02016(1)i(i+1)(i+2)=1i=22016(1)iiln(2) \lim_{n \rightarrow \infty} \sum_{i=0}^{2016} (-1)^i \cdot \frac{\binom{n}{i} \binom{n}{i+2}}{\binom{n}{i+1}^2} = \sum_{i=0}^{2016} (-1)^i \cdot \frac{(i+1)}{(i+2)} = 1 - \sum_{i=2}^{2016} \frac{(-1)^i}{i} \approx \ln(2)
Then 1A1ln(2)1.44\frac{1}{A} \approx \frac{1}{\ln(2)} \approx 1.44, so the answer is 11 .

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