Krishna
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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.