已知正方体\(ABCD-A_{1}B_{1}C_{1}D_{1}\) ，\(E\)，\(F\)分别是\(AA_{1}\)，\(CC_{1}\)的中点

\((1)\)求证：四边形\(BEDF_{1}\)为平行四边形

\((2)\)求异面直线\(BE\)与\(AC\)的夹角的余弦值