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  



        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.