OD = OB OC = OC

∠ ABC = 90 = ∠ CDO (the line between the tangent point of a straight line and a circle and the center of the circle is perpendicular to the straight line)

∴RT△ODC≌RT△OBC

BC =BD

(2)

∵RT△ODC≌RT△OBC

∴CD=CB

∴∠CDB=∠CBD

∠∠CDO = 90°

∴∠ADE+∠CDB=90

∠ABC = 90°。

∴∠ABD+∠CBD=90

∴∠ADE=∠ABD

(3)

∫AD is tangent to the circle at point d.

∴AD square = =AE×AB

∴2*2= 1*AB

AB=4

EB=3

The diameter of this circle is 3.