Let O be the centre of the circle.
By tangent chord theorem∠CAB =∠BCD=30°
Also ∠OCD=90°(Angle between tangent and radius is 90 degree)
∴∠BCO=90°−30°=60°. Also ∠CBO=60° because,Angle opposite to equal sides are equal.
And hence ΔOBC must be a Equilateral triangle ∴∠COB=60°.
Also in Right Triangle OCD, ∠CDO=90°−60°=30°
So that BC=BD