Question

如图2-4-9,CA为⊙O的切线,切点为A,点B在⊙O上,如果∠CAB=55°,那么∠AOB等于(    )

图2-4-9

A.55°                 B.90°               C.110°                D.120°

Answer 1

解析:延长AO交⊙O于D,连结BD,

∵AC切⊙O于A,AB是弦,

∴∠D=∠CAB.

又∵∠D=∠AOB,

∴∠AOB=2∠CAB=110°.

答案:C