sin20+sin40-sin80=???
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sinC+sinD=2sin(C+D/2)cos(C-D/2)
sin20+sin40=2 sin(20+40/2)cos(40-20/2)
2sin30cos10=2×1/2cos10=cos10
cos10-sin80=sin(90-10)-sin80
sinC-sinD=2 cos(C+D)/2sin(C-D)/2
2cos(90-10+80)/2sin(90-10-80)/2
=2cos160/2sin0=0
sin20+sin40=2 sin(20+40/2)cos(40-20/2)
2sin30cos10=2×1/2cos10=cos10
cos10-sin80=sin(90-10)-sin80
sinC-sinD=2 cos(C+D)/2sin(C-D)/2
2cos(90-10+80)/2sin(90-10-80)/2
=2cos160/2sin0=0
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