Hello,Originally Posted by totalnewbie
because AB = BC then AB = BD and BC = BD because the Points A, C, D are points of the semicircle of Thales. B is the centre of this circle.
Triangle ADB is an isosceles triangle, therefore the angle at A has a value of 30°.
The angle at D is a right one. Therefore the angle at C has a value of 60°.
Hope that this helps a little bit.