Maths Olympiad Prep

Library / /828 of 860

Algebra Difficulty 5.7 AIME, harder Find the answer

Compute k=11007(cos(πk1007))2014\sum_{k=1}^{1007}\left(\cos \left(\frac{\pi k}{1007}\right)\right)^{2014}.

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

Solution

Our desired expression is 122014k=11007(ωk+ωk)2014\frac{1}{2^{2014}} \sum_{k=1}^{1007}\left(\omega^{k}+\omega^{-k}\right)^{2014}. Using binomial expansion and switching the order of the resulting summation, this is equal to 122014j=02014(2014j)k=11007(ω20142j)k\frac{1}{2^{2014}} \sum_{j=0}^{2014}\binom{2014}{j} \sum_{k=1}^{1007}\left(\omega^{2014-2j}\right)^{k}. Note that unless ω20142j=1\omega^{2014-2j}=1, the summand k=11007(ω20142j)k\sum_{k=1}^{1007}\left(\omega^{2014-2j}\right)^{k} is the sum of roots of unity spaced evenly around the unit circle in the complex plane (in particular the 1007th, 19th, and 53rd roots of unity), so it is zero. Thus, we must only sum over those jj for which ω20142j=1\omega^{2014-2j}=1, which holds for j=0,1007,2014j=0,1007,2014. This yields the answer 122014(1007+1007(20141007)+1007)=2014(1+(20131007))22014\frac{1}{2^{2014}}\left(1007+1007\binom{2014}{1007}+1007\right)=\frac{2014\left(1+\binom{2013}{1007}\right)}{2^{2014}}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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