矩形\(ABCD\)中,\(AD\)\(=5\),\(AB\)\(=3\),将矩形\(ABCD\)沿某直线折叠,使点\(A\)的对应点\(A\)\(′\)落在线段\(BC\)上,再打开得到折痕\(EF\).
\((1)\)当\(A\)\(′\)与\(B\)重合时\((\)如图\(1)\),\(EF\)\(=\) ;当折痕\(EF\)过点\(D\)时\((\)如图\(2)\),求线段\(EF\)的长;
\((2)\)观察图\(3\)和图\(4\),\(①\)利用图\(4\),证明四边形\(AEA\)\(′\)\(F\)是菱形;\(②\)设\(BA\)\(′=x\),当\(x\)的取值范围是 时,四边形\(AEA\)\(′\)\(F\)是菱形.