原式=cos50(1+√3*sin10/cos10)
=cos50*(√3sin10+cos10)/cos10
=cos50*2sin(10+a)/cos10
其中tana=1/√3,所以此处a=30
所以原式=2cos50sin40/cos10
=2sin(90-50)*sin40/sin(90-10)
=2(sin40)^2/sin80
=2(sin40)^2/2sin40cos40
=sin40/cos40
=tan40