∵AB and ⊙ are tangent to point C, so OC⊥AB (tangent property theorem)

∵OA=OB, ∴ in isosceles triangle, AC=BC= 1/2AB=5cm (isosceles triangle property).

In Rt△ACO, AOˇ2=OCˇ2+ACˇ2 (Pythagorean theorem).

∴aoˇ2=4ˇ2+5ˇ2 = 16+25 = 4 1

The square root of ∴ ao = 4 1 ˇ (1/2)-41is difficult to be expressed by symbols here, so it is expressed by the (1/2) power of 41.

This problem can also be calculated by cutting lines, which will be more intuitive. You can try.