(20 10? Nanjing) As shown in the figure, in two concentric circles with O as the center, the chord AB of the great circle is the tangent of the small circle, and C is the tangent point. If the radii of two circles are 3 cm respectively, Solution: connect OC, OA,

Ab cuts from o to c,

∴OC⊥AB，

∴ab=2ac；

∵ At Rt△OAC, OA=5cm, OC=3cm,

∴AC=OA2？ OC2 = 4cm,

∴AB=2AC=8cm.