ISOSCELES TRIANGLE,Cosine rule in ΔBCD, –⇒ BC2 + DC2 = BD2 + 2 × BC × DC × cos120° [∵ all internal angles of HEXAGON is 120°]⇒ a2 + a2 = BD2 – 2 × a × a × cos60° [∵ cos(180° – 60°) = -cos 60°]⇒ BD2 = 3a2⇒ BD = √3aNow consider ΔABD,⇒ tan∠ADB = AB/BD⇒ tan∠ADB = a/√3a = 1/√3⇒ tan∠ADB = tan 30°∴ ∠ADB = 30°