求值cos10°分之sin20°cos20°cos40°.

分类: 作业答案 • 2021-12-20 01:20:39

问题描述：

答

原式(2cos10-sin20)/cos20 =(cos10+cos10-sin20)/cos20 =[cos10+(cos10-cos70)]/cos20 =[cos10+cos(40-30)-cos(40+30)]/cos20 =[

答

sin20°cos20°cos40°/cos10°
=(1/2)sin40°cos40°/cos10°
=(1/4)sin80°/cos10°
=(1/4)sin80°/sin80°
=1/4