给解题思路:
∠ABD=120°
AB是⊙O的切线 ---- ∠ABO = 90°
∠OBD = ∠ABD - ∠ABO = 30°
O为圆心 -- OB = OD -- ∠OBD = ∠ODB = 30°
∠BAD = 180 - ∠ABD - ∠ODB = 30
OB = 1/2 AO
OC = OB
OC= 1/2 AO
C为AO中点
AC = OC
给解题思路:
∠ABD=120°
AB是⊙O的切线 ---- ∠ABO = 90°
∠OBD = ∠ABD - ∠ABO = 30°
O为圆心 -- OB = OD -- ∠OBD = ∠ODB = 30°
∠BAD = 180 - ∠ABD - ∠ODB = 30
OB = 1/2 AO
OC = OB
OC= 1/2 AO
C为AO中点
AC = OC