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(1)当a=
![manfen5.com 满分网](http://img.manfen5.com/res/CZSX/web/STSource/20131103102357558461567/SYS201311031023575584615008_ST/0.png)
,b=2时,求
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的值;
(2)如图,在⊙O中,AB是直径,∠BOC=120°,PC是⊙O的切线,切点是C,点D在劣弧BC上运动.当∠CPD满足什么条件时,直线PD与直线AB垂直?证明你的结论.
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