(1)证明:AC⊥BD;
(2)已知△ACD是直角三角形,AB=BD.若E为棱BD上与D不重合的点,且AE⊥EC,求四面体ABCE与四面体ACDE的体积比.
(Ⅰ)证明: ;
(Ⅱ)证明: ;
(Ⅲ)求三棱锥的体积.