Question

如图2-6,PA为⊙O的切线,A为切点,PA=8,PCB是割线,交圆于C、B两点,且PC=4,AD⊥BC于D,∠ABC=α,∠ACB =β,连结AB、AC,则的值等于 …(　　)

图2-6

A.                     B.                         C.2                       D.4

Answer 1

思路解析:从入手考察.?

∵AD⊥BC,∴=·=.?

由条件容易发现△PAC∽△PBA,从而=.?

由切割线定理容易求得PB.?

∵PA² =PC·PB,?

∴PB = = =16.?

∴= =.

答案:B