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Algebra Difficulty 5.1 AIME, harder Find the answer

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}.

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

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 1 .

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