如图,四角棱锥p-ABCD的底面ABCD是∠BAD=60°的平行四边形,平面pDC⊥平面ABCD,pD=pC=5,AB=8,BC=3.&nBsp;
(1)求pA与BC所成角的余弦值；&nBsp;
(2)求二面角p-AD-C的正弦值.&nBsp;