cos(α-β)=cosα·cosβ+sinα·sinβ
sin(2α)=2sinα·cosα=2/(tanα+cotα)
cos(2α)=cos^2(α)-sin^2(α)=2cos^2(α)-1=1-2sin^2(α)
tan(2α)=2tanα/[1-tan^2(α)]
原式
=2(cos60*cos10+sin60*sin10)*tan70-2cos40
=2(cos50)*tan70-2cos40
=2sin40*cos20/sin20-2cos40
=4sin20*cos20*cos20/sin20-2[cos^2(20)-sin^2(20)]
=4sin^2(20)-2cos^2(20)+2sin^2(20)
=-2[cos^2(20)+sin^2(20)]
=-2