sin4x=cosx2sin2xcos2x=cosx4sinxcosx(2cosxcox-1)/2=cosx2sinxcosx(2cosxcosx-1)=cosx[1]cosx=0,x=圆周率pi/+kpi[2]sinx=0,x=kpi[3]2cosxcosx-1=0cosx=正负二分之根二,x=pi/4+kpi/2