求值cos20°*cos40°*cos80° - 知识问答

求值cos20°*cos40°*cos80°

2个回答

原式=2sin20cos20cos40cos80/2sin20
=sin40cos40cos80/2sin20
=2sin40cos40cos80/4sin20
=sin80cos80/4sin20
=2sin80cos80/8sin20
=sin160/8sin20
=sin(180-160)/8sin20
=sin20/8sin20
=1/8

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