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如图,四棱锥P-ABCD中,PA⊥平面ABCD,AD∥BC,AB=AD=AC=3,PA=BC=4,M为线段AD上一点,AM=2MD,N为PC的中点。

(Ⅰ)证明:MN∥平面PAB

(Ⅱ)求直线AN与平面PMN所成角的正弦值。点,AM=2MD,N为PC的中点。

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