∵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. ..