Step 1: Draw a circle and diameter.

Step 2: Sketch the tangent through the end points of the diameter

Step 3: Recall the relationship between the radius and tangent.

NOTE: Tangent at any point of circle perpendicular to the radius through

point of contact.

Step 4: Find the alternating interior angles of the two tangents

NOTE: Two tangents are parallel because the alternating interior angles are

same.

EXAMPLE: O - center, AB - diameter and PQ and RS are tangents.

then, OB perpendicular to RS

OA perpendicular to PQ

Alternating interior angles

\angle OAQ = \angle OBR = 90

\angle OAP = \angle OBS = 90

Since PQ and RS are parallel lines.