(I )求证:平面PAB丄平面PCD;
(II)如果AB=BC,PB=PC,求二面角B﹣PC﹣D的余弦值.
⑴____A. shop B. stop C. no
⑵____A. can B. cake C. sad
⑶____A. Thursday B. hurt C. nurse
⑷____A. time B. bike C. kite
⑸____A. bus B. us C. put