sin(18°)sin(54°)sin(18°)*sin(54°) 不用计算器
问题描述：
sin(18°)sin(54°)
sin(18°)*sin(54°) 不用计算器
答
sin(54°)=cos(36°)
原式=[sin(18°)*cos(36°)]/1 分母是1
=[cos(18°)*sin(18°)*cos(36°)]/cos(18°)
=[sin(36°)*cos(36°)]/[2cos(18°)]
=[sin(72°)]/[4cos(18°)]
=[cos(18°)]/[4cos(18°)]
=1/4
答
cos36*cos72
=cos36*sin18
=(1/cos18)*sin18*cos18*cos36
=(1/2cos18)*sin36*cos36
=(1/4cos18)*sin72
=1/4