cos(-20°)+cos140°+cos100°=0的解答过程是.

问题描述:

cos(-20°)+cos140°+cos100°=0的解答过程是.

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