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OA = OC，

∴∠OCA=∠OAC.

∫ communication split ∠ bad,

∴∠BAC=∠CAD.

∴∠OCA=∠CAD.

∴OC∥AD.

And ∵AD⊥CD,

∴OC⊥CD.

∴CD is the tangent of⊙ O (4 points).

(2) solution: connect BC,

∫AB is the diameter,

∴∠BCA=90。

∴∠BCA=∠ADC=90。

∫∠BAC =∠CAD，

∴△BAC∽△CAD.

∴ ABAC = acad, that is, 4AC=AC3. ..

∴AC=23.

In Rt△ABC, cos ∠ BAC = acab = 32.

∴∠BAC=30。