求值sin42°-cos12°+sin54°
答案:1 悬赏:30 手机版
解决时间 2021-12-03 19:08
- 提问者网友：孤凫
- 2021-12-03 13:58
求值sin42°-cos12°+sin54°
最佳答案
- 五星知识达人网友：不如潦草
- 2021-12-03 14:11
sin42°-cos12°+sin54°
=sin42°-sin78°+sin54°
=-2cos60°sin18°+sin54°
=sin54°-sin18°
=2cos36°sin18°
=2cos36°sin18°sin36°/sin36°
=sin18°sin72°/sin36°
=2sin18°cos18°/2sin36°
=sin36°/2sin36°
=1/2
=sin42°-sin78°+sin54°
=-2cos60°sin18°+sin54°
=sin54°-sin18°
=2cos36°sin18°
=2cos36°sin18°sin36°/sin36°
=sin18°sin72°/sin36°
=2sin18°cos18°/2sin36°
=sin36°/2sin36°
=1/2
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