cosx+cos2x+cos3x+.+cosnx
=sin(x/2)*[ cosx+cos2x+cos3x+.+cosnx] / sin(x/2) （ 将sin(x/2) 移入方括号里并化简）
= {sin[x(2n+1)/2] - sin(x/2) }/ [2sin(x/2)]