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
求值sin42°-cos12°+sin54°
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