高中数学 如图15题
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=(tan60° - tan80°)/cos10°
=(sin60°/cos60° - sin80°/cos80°)/cos10°
=(sin60°cos80° - cos60°sin80°)/(cos60°cos80°cos10°)
=sin(60°-80°)/(cos60°*sin10°*cos10°) 注：cos80°=sin10°
=sin(-20°)/[cos60° * (sin10°*cos10°)]
=-sin20°/[cos60° * (2sin10°*cos10°)/2]
=-2sin20°/[cos60° * sin20°] 注：sin20° =2sin10°*cos10°
=-2/cos60°
=-2/(1/2)
=-4
=(sin60°/cos60° - sin80°/cos80°)/cos10°
=(sin60°cos80° - cos60°sin80°)/(cos60°cos80°cos10°)
=sin(60°-80°)/(cos60°*sin10°*cos10°) 注：cos80°=sin10°
=sin(-20°)/[cos60° * (sin10°*cos10°)]
=-sin20°/[cos60° * (2sin10°*cos10°)/2]
=-2sin20°/[cos60° * sin20°] 注：sin20° =2sin10°*cos10°
=-2/cos60°
=-2/(1/2)
=-4
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