求值:(2cos80°+cos160°)/cos70°

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求值:(2cos80°+cos160°)/cos70°求值:(2cos80°+cos160°)/cos70°求值:(2cos80°+cos160°)/cos70°cos160°+cos80°=cos(

求值:(2cos80°+cos160°)/cos70°
求值:(2cos80°+cos160°)/cos70°

求值:(2cos80°+cos160°)/cos70°
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°)/cos70° = -2sin60° = -根号3