(本小题满分12分)如图:在底面为直角梯形的四棱锥P-ABCD中,AD‖BC ,∠ABC=90°,PA⊥平面ABCD, PA="3," AD="2," AB=, BC=6.(1)求证:BD⊥平面PAC(2)求二面角B-PC-A的大小.