From an external point P, two tangents PA and PB are drawn to a circle with centre O. The distance OP is 13 cm. Another point Q lies on the same line OP produced such that OQ = 25 cm. Tangents QC and QD are drawn to the same circle.
(i) If QC is exactly twice PA, find the radius of the circle.
(ii) Hence, find the common length of the tangents PA and PB drawn from P.