∵BC is the tangent of∵ O, ∴∠ CBA = 90, ∴∠D=∠CBA.

And ∵ad∨CD, ∴∠A=∠COB,

∴△adb∽△obc；

(2)∫AB = 2，OB= 1，

Adbd = obbc = 12 from ( 1)，

In Rt△ADB, AD2+BD2=AB2.

∴AD2+(2AD)2=22，

Because AD > 0, the solution is AD=233. ..