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How to write 14, 20, 23, the mathematics test questions of Anhui senior high school entrance examination in 2006?
: ∵BC is the tangent of ⊙O,

∴∠ABC=90,

∵OD⊥AC,

∴∠ Addo = 90 degrees,

∴△ABC∽△ADO,

∴ODBC

=AO

Alternating current

That is, OD=AO×BC.

Alternating current

In △ABC,

∠BAC=30,

∴AC=2BC=8 3

AB= AC 2

In 2 BC

= 12,

∴OA=6=BO,

∴OD=AO×BC

Alternating current

=6×4 3

8 3

=3.