(2014•唐山三模)在斜三棱柱ABC-A1B1C1中,平面A1ACC1⊥平面ABC,AC⊥BC,A1B⊥C1C,AC=BC.
(1)求证A1A⊥A1C;
(2)若A1A=A1C,求二面角B-A1C-B1的余弦值.