原式=cos20°cos10°/sin20°-√3sin10°cos20°/sin20°-2cos40°
=(cos10°-√3sin10°)cos20°/sin20°-2cos40°
=2cos(10°+60°)cos20°/sin20°-2cos40°
=2sin20°cos20°/sin20°-2cos40°
=2cos20°-2cos40°
=2(cos20°-cos40°)
=4sin60°sin(-20°)
=-2√3sin20°