Two external points P and Q lie on the same straight line through the centre O of a circle, with Q being farther from O than P. The distance OP is 15cm, and the tangent drawn from P to the circle is 9cm long. The tangent drawn from Q to the same circle is 12cm long.
(i) Find the radius of the circle.
(ii) Determine the distance OQ.
(iii) Hence calculate the length PQ.
c. (iii) Calculate the length PQ.