求值：（2cos80°+cos160°)/cos70°
问题描述：
求值：（2cos80°+cos160°)/cos70°
答
cos160° + cos80° = cos(120°+40°) + cos(120°-40°) = 2cos120°cos40° = -cos40°cos80° - cos40° = cos(60°+20°) - cos(60°-20°) = -2sin60°sin20° = -2sin60°cos70°所以（2cos80°+cos160°)/co...