tan6π9−33tan4π9+27tan2π9
Assuming tanπ9=t
tan6π9−33tan4π9+27tan2π9=t6−33t4+27t2 ⋯(1)
Since, tan3θ=3tanθ−tan3θ1−3tan2θ
Putting θ=π9, we get
tanπ3=3tanπ9−tan3π91−3tan2π9⇒√3=3t−t31−3t2⇒√3=t(3−t2)1−3t2⇒√3(1−3t2)=t(3−t2)
Squaring on both sides,
3(1+9t4−6t2)=t2(9+t4−6t2)⇒3=t6−33t4+27t2
From equation (1),
tan6π9−33tan4π9+27tan2π9=3