(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=...