A:when you bought this MP5
B:how long you have had this MP5
C:how soon you will buy a new MP5
D:where you bought this MP5
如图,在四棱锥P﹣ABCD中,PA⊥平面ABCD,底面ABCD是菱形.
(1)求证:BD⊥平面PAC;
(2)若PA=AB,求PB与AC所成角的余弦值.
在①0<cosα<1(α为锐角)②sin62°<cos62°③tan45°<sin89°④sin35°=cos55°;其中正确的是( )