On the Mathematical Problems of Rings
Solution: connect oc, ob and oa. Because AB cuts a small circle, OC is perpendicular to AB. Because the area of the circle =9π, πR? 0? 5-πr？ 0? 5=9π R？ 0? 5-r？ 0? 5=9 AC in right triangle OCA? 0? 5= R？ 0? 5-r？ 0? So AC=3, AC=CB, so AB=6.