越快越好.cos40·xcos80·xcos160 - 知识问答

越快越好.cos40·xcos80·xcos160·

3个回答

=2sin40cos40cos80cos160/(2sin40)
=sin80cos80cos160/(2sin40)
=sin160cos160/(4sin40)
=sin320/(8sin40)
=-sin40/(8sin40)
=-1/8

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