4.
\((1)\)如果\(OA/\!/O′A′\),\(OB/\!/O′B′\),那么\(∠AOB\)和\(∠A′O′B′\)的关系为____________
\((2)\)圆锥的表面积是底面积的\(3\)倍,那么该圆锥的侧面展开图扇形的圆心角的大小为____________.
\((3)\)如图,在棱长为\(a\)的正方体\(ABCD-A_{1}B_{1}C_{1}D_{1}\)中,\(O\)为底面\(ABCD\)的中心,点\(E\),\(F\)分别是棱\(CC_{1\;,}AD\)的中点,则异面直线\(OE\)和\(FD_{1}\)所成角的余弦值为__________.
\((4)\)如图所示,在棱长为 \(6\)的正方体\(ABCD-A_{1}B_{1}C_{1}D_{1}\)中,点\(E\),\(F\)分别是棱\(C_{1}D_{1}\),\(B_{1}C_{1}\)的中点,过\(A\),\(E\),\(F\)三点作该正方体的截面,则截面的周长为_________________.