(本题满分12分)如图,底面为菱形的四棱锥P-ABCD中,∠ABC=60°,AC="1," PA="2," PB=PD= ,点M是PD的中点. (Ⅰ)证明:PA⊥平面ABCD;(Ⅱ)若AN为PD边的高线,求二面角M-AC-N的余弦值.