we know:-

tan(A - B) = {tanA - tanB} / (1 + tanAtanB)-----( 1 )

tan(A + B) = {tanA + tanB} / (1 - tanAtanB)-----( 2 )

multiply--------( 1 ) & ------( 2 )

tan(A - B)*tan(A + B) = {tanA - tanB} / (1 + tanAtanB) * {tanA + tanB} / (1 - tanAtanB)

tan(A - B)*tan(A + B) = (tan²A - tan²B) / (1 - tan²A tan²B)

put A = 60° and B = {\Theta} we get,

tan(60 - {\Theta})tan(60 + {\Theta}) = \bold{\frac{(\sqrt{3})^2 - tan^2{\Theta}}{1 - (\sqrt{3})^2tan^2{\Theta}}}

tan(60 - {\Theta})tan(60 + {\Theta}) = \bold{\frac{3 - tan^2{\Theta}}{1 - 3tan^2{\Theta}}}

tan(60 - {\Theta})tan(60 + {\Theta}) = \frac{tan\Theta}{tan\Theta} * \bold{\frac{3 - tan^2{\Theta}}{1 - 3tan^2{\Theta}}}

tan(60 - \Theta)tan(60 + \Theta) = \bold{\frac{1}{tan\Theta}} * \bold{\frac{3tan\Theta - tan^3\Theta}{1 - 3tan^2\Theta}}

we know :- tan3A = \bold{\frac{3tan\Theta - tan^3\Theta}{1 - 3tan^2\Theta}}

tan\Thetatan(60 - \Theta)tan(60 + \Theta) = tan3\Theta----------( 3 )

put \Theta = 20, in ---------( 3 )

tan(20) tan(60 - 20) tan(60 + 20) = tan(3*20)

multiply both side by "tan60"

tan(20) tan(40) tan(60) tan(80) = tan(60) * tan(60)

tan(20) tan(40) tan(60) tan(80) = (√3) * (√3)

tan(20) tan(40) tan(60) tan(80) = 3

❤❤❤❤ Hope you will understand....as well as I hope it was helpful to u.....