⑴连接OT,∵T为切点,∴OT⊥AC,
连接OD,∵OB=OD,∴∠OBD=∠ODB,
∵AB=AC,∴∠C=∠ABC,
∵∠OBD=∠ABC,
∴∠C=∠ODB,
∴OD∥AC,
∴OD⊥OT,
∵DE⊥AC,∴OD⊥DE,
∴DE是⊙O的切线.
⑵由⑴知,四边形ODET是矩形,
∴ET=OD=3,设AB=AC=X,
CT=9-3=6,
∴AT=6-X,AO=3+X,
在RTΔATO中,AO^2=AT^2+OT^2,
∴(3+X)^2=(6-X)^2+9
X=2,
∴AB=2.
⑴连接OT,∵T为切点,∴OT⊥AC,
连接OD,∵OB=OD,∴∠OBD=∠ODB,
∵AB=AC,∴∠C=∠ABC,
∵∠OBD=∠ABC,
∴∠C=∠ODB,
∴OD∥AC,
∴OD⊥OT,
∵DE⊥AC,∴OD⊥DE,
∴DE是⊙O的切线.
⑵由⑴知,四边形ODET是矩形,
∴ET=OD=3,设AB=AC=X,
CT=9-3=6,
∴AT=6-X,AO=3+X,
在RTΔATO中,AO^2=AT^2+OT^2,
∴(3+X)^2=(6-X)^2+9
X=2,
∴AB=2.