cos(2pi/7)+cos(4pi/7)+cos(6pi/7)
=1/[2sin(2pi/7)]*[2sin(2pi/7)cos(2pi/7)+2sin(2pi/7)cos(4pi/7)+2sin(2pi/7)cos(6pi/7)]
进行积化和差
=1/[2sin2pi/7)]*{sin(4pi/7)+[sin(6pi/7)-sin(2pi/7)] +[sin(8pi/7)-sin(4pi/7)]}
=1/[2sin(2pi/7)][-sin(2pi/7)+sin(6pi/7)-sin(6pi/7)]
=1/[2sin(2pi/7)]*[-sin(2pi/7)]
=-1/2.