In the figure, BC is a chord and CD is a tangent through the point C.  If ∠AOC = 138°, then find ∠ACD.

42° 69° 21° 48°

69°

∠ABC = \(\frac{∠AOC}{2}\) = \(\frac{138}{2}\) = 69° Angle formed on center is double the angle formed on circumference by same arc. ∠ABC = ∠ACD = 69° (alternate segment theorem)