6.
如图,在四棱锥\(P-ABCD\)中,底面\(ABCD\)是正方形,\(AD=PD=2,PA=2\sqrt{2},\)
\(\angle PDC={{120}^{\circ }}\),点\(E\)为线段\(PC\)的中点,点\(F\)在线段\(AB\)上\(.\)
\((\)Ⅰ\()\)若\(AF=\dfrac{1}{2}\),求证:\(CD\bot EF\);
\((\)Ⅱ\()\)设平面\(DEF\)与平面\(DPA\)所成二面角的平面角为\(\theta \),试确定点\(F\)的位置,使得\(\cos \theta =\dfrac{\sqrt{3}}{4}\).