COS(PAI/5)COS(2PAI/5)=?XIE

来源：学生作业帮助网编辑：作业帮时间：2024/05/05 00:30:12

COS(PAI/5)COS(2PAI/5)=?XIE
COS(PAI/5)COS(2PAI/5)=?
XIE

COS(PAI/5)COS(2PAI/5)=?XIE
=(2sinπ/5cosπ/5cos2π/5)/2sinπ/5
=(sin2π/5cos2π/5)/2sinπ/5
=(2sin2π/5cos2π/5)/4sinπ/5
=sin4π/5/4sinπ/5
=sin(π-π/5)/4sinπ/5
=sinπ/5/4sinπ/5
=1/4

在前面*sin(pi/5)
原式=1/2（2*sin(pi/5)*cos(pi/5)*cos(2pi/5)）/sin(pi/5)
=1/4(2sin(2pi/5)*cos(2pi/5))/sin(pi/5)
=1/4(sin(4pi/5))/sin(pi/5)=1/4
其中 sin(4pi/5)=sin(pi/5)
圆周率英文是写作pi的

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