cos（-20°）+cos140°+cos100° - 知识问答

cos（-20°）+cos140°+cos100°=0的解答过程是.

2个回答

cos（-20°）+cos140°+cos100°
=cos20°-(cos40°+cos80°)
=cos20°-2cos[(40°+80°)/2]cos(80°-40°)/2]
=cos20°-2cos60°cos20°
=cos20°-cos20°
=0=右边.

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