Angle between the tangent and its chord is equal to the angle in the alternate segment.
Proof:
Construct the diameter AD, and Join DB
Δ ADB is a right triangle as it is in a semicircle, ∠VBA = 90°
∠ADB = 180°−90°−∠DAB
∠ADB = 90°−∠DAB
∠ADB = 90°−∠SAB tangent and diameter
∠ADB = ∠ACB angle subtended by the same arc
∠ACB = ∠SAB
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