计算：sin20°-sin40°/cos20°-cos40°

sin20°-sin40°/cos20°-cos40°
=sin20°-sin40°/sin70°-sin50°
=2sin[(20-40)/2]cos[(20+40)/2]/2sin[(70-50)/2]cos[(70+50)/2]
=sin(-10)cos30/sin10cos60
=-sin10cos30/sin10cos60
=-sin60/cos60
=--tan60
=-√3