AlgebraDifficulty 5.8AIME, harderProve itUnited States
Problem: Let A=n→∞limi=0∑2016(−1)i⋅(i+1n)2(in)(i+2n) Find the largest integer less than or equal to A1. The following decimal approximation might be useful: 0.6931<ln(2)<0.6932, where ln denotes the natural logarithm function.
Solution
Solution: Note i=0∑2016(−1)i⋅(i+1n)2(in)(i+2n)=i=0∑2016(−1)i⋅(i+2)(n−i)(i+1)(n−i−1) So n→∞limi=0∑2016(−1)i⋅(i+1n)2(in)(i+2n)=i=0∑2016(−1)i⋅(i+2)(i+1)=1−i=2∑2016i(−1)i≈ln(2) Then A1≈ln(2)1≈1.44, so the answer is 1 .
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