如图,在正四棱柱\(ABCD-A_{1}B_{1}C_{1}D_{1}\)中,\(AA_{1}= \dfrac {1}{2}AB\),点\(E\)、\(M\)分别为\(A_{1}\)B、\(C_{1}C\)的中点,过点\(A_{1}\)、\(B\)、\(M\)三点的平面\(A_{1}BMN\)交\(C_{1}D_{1}\)于点\(N\).
\((1)\)求证:\(EM/\!/\)平面\(A_{1}B_{1}C_{1}D_{1}\);
\((2)\)求二面角\(B-A_{1}N-B_{1}\)的正切值;
\((3)\)设截面\(A_{1}BMN\)把该正四棱柱截成的两个几何体的体积分别为\(V_{1}\)、\(V_{2}(V_{1} < V_{2})\),求\(V_{1}\):\(V_{2}\)的值.