Theorem: (Tangent- Secant theorem or Alternate Segment theorem)
The angle between chord AB and tangent PQ through the point of contact is equal to the angle in the alternate segment.
Given: PQ is a tangent at A to the circle with O as center. AB is a chord.
To prove: If C is a point on a major arc and D is a point on a minor arc with respect to the chord AB then
BAQ = ACB and PAB = ADB
Construction: Join OB