The diameter of circle C is 18 cm. mAD is 60°. Answer the following: 

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Been awhile for geometery, but m<DAB = 90º.  A is on the circle ... arc BD is 180º since BD is the diameter. Angle is ½ the arc. m<DBA = 30º ... ½ the arc length of arc DA. m<BDA = 60º ... triangle adds to 180º in addition ½ the arc length remaining. Since a Rt Triangle... trig works (or faster if you know the 30-60-90 triangle relationships! sin <ABD = L/18 L  = 18 (sin 30) L = 18(½) L = 9 cm