求值：sin21°cos81°-sin69°cos - 知识问答

求值：sin21°cos81°-sin69°cos9°

2个回答

sin21cos81-sin69cos9
=-sin21sin69cos81cos9
=[-sin21cos(90-69)][sin(90-81)cos9]
=(-sin21cos21)(sin9cos9)
=-(1/2sin42)(1/2sin18)
=-1/4sin42sin18
额,后面的忘了,自己算算吧,翻翻书,有这种类型的解法的.

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