tan60°=√3
(2cos10°-sin20°)/cos20°
=(2cos(30°-20°)-sin20°)/cos20°
=[2(cos30cos20+sin30sin20)-sin20°]/cos20°
=(√3cos20+sin20-sin20°)/cos20°
=√3cos20/cos20°=√3
tan60°(2cos10°-sin20°)/cos20°=√3*√3=3