急急急！在线等！一道数学题，一定要有步骤！

显示全部楼层 · 2012-8-4 09:52:30

cos(4派/5)=-cos(派/5),cos(3派/5)=-cos(2派/5).所以抵消就是0...

千问 · 2012-8-4 09:52:30

cos(派/5)加cos(2派/5)加cos(3派/5)加cos(4派/5) =【cos(派/5)加cos(4派/5) 】+【cos(2派/5)加cos(3派/5)】=【cos(派/5)+cos(派-派/5)】+【cos(2派/5)加cos(派-2派/5)】=0+0=0...

千问 · 2012-8-4 09:52:30

cos(派/5)加cos(2派/5)加cos(3派/5)加cos(4派/5)=cos(1/5+2/5+3/5+4/5)派=cos2派...

千问 · 2012-8-4 09:52:30

直接用计算器算出每一个数的值加起来就可以了么...

千问 · 2012-8-4 09:52:30

cosπ/5＋cos2π/5＋cos3π/5＋cos4π/5＝sin﹙π/2-π/5﹚＋sin﹙π/2-2π/5﹚＋sin﹙π/2-3π/5﹚＋sin﹙π/2-4π/5﹚＝sin3π/10＋sinπ/10－sinπ/10－sin3π/10＝0...