如图,在三棱锥\(P-ABC\)中,\(PA⊥\)平面\(ABC\),\(AC⊥BC\),\(D\)为\(PC\)的中点,\(E\)为\(AD\)的中点,点\(F\)在线段\(PB\)上,\(PA=AC=4\),\(BC=2\).
\((\)Ⅰ\()\)求证:\(AD⊥\)平面\(PBC\);
\((\)Ⅱ\()\)若\( \dfrac {PF}{PB}= \dfrac {3}{4}\),求证:\(EF/\!/\)平面\(ABC\);
\((\)Ⅲ\()\)求\(PE\)与平面\(ADB\)所成角的正弦值.