Evaluate the following integral \[ \int_0^\infty \frac{\cos cx}{(\cosh x)^2} \, dx\] for a real constant \(c\).

The best solution was submitted by Sunghyuk Park (박성혁, 수리과학과 2014학번). Congratulations!

Here is his solution of problem 2015-22.

Alternative solutions were submitted by 신준형 (2015학번, +3), 이종원 (수리과학과 2014학번, +3), 장기정 (수리과학과 2014학번, +3), 유찬진 (2015학번, +2), 최인혁 (2015학번, +2), 이예찬 (오송고등학교 교사, +2), Luis F. Abanto-Leon (+2).