如图,\(\triangle ABC\)中,\(O\)是\(BC\)的中点,\(AB=AC\),\(AO=2OC=2.\)将\(\triangle BAO\)沿\(AO\)折起,使\(B\)点与图中\(B{{'}}\)点重合.
\((\)Ⅰ\()\)求证:\(AO⊥\)平面\(B′OC\);
\((\)Ⅱ\()\)当三棱锥\(B{{'}}-AOC\)的体积取最大时,求二面角\(A-B′C-O\)的余弦值;
\((\)Ⅲ\()\)在\((\)Ⅱ\()\)的条件下,试问在线段\(B′A\)上是否存在一点\(P\),使\(CP\)与平面\(B′OA\)所成的角的正弦值为\( \dfrac {2}{3}\)?证明你的结论.