sin α-sin β=2cos[(α+β)/2]×sin[(α-β)/2]
cos α-cos β=-2sin[(α+β)/2]×sin[(α-β)/2]
(sin20°-sin40°)÷(cos20°-cos40°)
= (sin40°-sin20°)÷(cos40°-cos20°)
= {2cos[(40+20)/2]×sin[(40-20)/2]} ÷{-2sin[(40+20)/2]×sin[(40-20)/2] }
= - cos30sin10÷sin30cos10
= -√3tan10