tan9 - tan27 - tan63 + tan 81 = ?
tan9−tan27−tan63−tan81tan9+tan81−tan27−tan63sin9/cos9+sin81/cos81−sin27/cos27−sin63/cos63sin90/cos81cos9−sin90/cos63cos271/sin9cos9−1/sin27cos272/sin18−2/sin54(2)sin54−sin18/sin18sin544cos36sin18/sin18cos36=4