如图,在四棱锥\(P-ABCD\)中,平面\(PAD⊥\)平面\(ABCD\),\(PA⊥PD\),\(PA=PD\),\(AB⊥AD\),\(AB=1\),\(AD=2\),\(AC=CD= \sqrt {5}\).
\((\)Ⅰ\()\)求证:\(PD⊥\)平面\(PAB\);
\((\)Ⅱ\()\)求直线\(PB\)与平面\(PCD\)所成角的正弦值;
\((\)Ⅲ\()\)在棱\(PA\)上是否存在点\(M\),使得\(BM/\!/\)平面\(PCD\)?若存在,求\( \dfrac {AM}{AP}\)的值,若不存在,说明理由.