圓形既某一題唔識= =

Assuming that the 3 circles touch each other.
1. PQ = distance between centre P and Q = radius of circle P + radius of circle Q
= PB + QC = 10 + 8 = 18. (Since AD is a tangent to the 2 circles).
2. OP = distance between centre O and centre P = radius of circle O - radius of circle P = OA - PB = 21 - 10 = 11.

2008-07-24 08:03:16 補充：
Note: If the 3 circles touch each other, it can be proved that PB should not be 10, it should be about 9.33.