Plane Geometry

(This problem is from CE Ref V4 of Gillesania)

A quadrilateral ABCD is inscribed in a semicircle with side AD as the diameter. If point O is the center of the diameter, determine the angle DCO if angle CAD is 39 deg.

In the book's solution, it says that angle COD = 2 times angle CAD. Is this a rule in semi circles?

(Attached is the drawn image of the problem)

Thanks in advance mga sir.

https://drive.google.com/open?id=0BzglifbmrqfKOUtqSFZEWnNYNVE

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