Radius will be perpendicular to these tangents.
Thus, OA ⊥ RS and OB ⊥ PQ
∠OAR = ∠OAS = ∠OBP = ∠OBQ = 90º
Therefore,
∠OAR = ∠OBQ(Alternate interior angles)
∠OAS = ∠OBP (Alternate interior angles)
Since alternate interior angles are equal, lines PQ and RS will be parallel.