1、26
2、65°,25°
3、6 cm
4 - 6 B B B
7、四边形ACDO为菱形.
8、∵ AB = BC = OB,
∴ 2∠C + 2∠O = 180°,
∴ ∠C + ∠O = 90°,
∴ ∠OAC = 90°,直线AC与⊙O相切.
9、延长AO交⊙O于E,连接BE,△ABE ∽ △ADC,
∴ ∠BAE = ∠DAC.
10、由CD = 2π3,得R = 2.连OC,OD,CD,△ACO ≌ △DCO,
∴ S阴影 = S扇形OCD = 2π³.
11、(1)∵ OC∥AB,
∴ ∠BAC = ∠OCA = ∠OAC;
(2)∵ AC∶CD = 2∶1,
∴ ∠D = 60°,
在Rt△ACD中,AD = ACsin60° = 1633,
∴ OA = 833