\((1)\)如图\(1\),在\(Rt\triangle ABC\)中,\(∠ACB=90^{\circ}\),\(CD⊥AB\)于点\(D\).
\(①\)如果\(AD=4\),\(BD=9\),那么\(CD=\)____________;
\(②\)如果以\(CD\)的长为边长作一个正方形,其面积为\({{s}_{1}}\),以\(BD\),\(AD\)的长为邻边长作一个矩形,其面积为\({{s}_{2}}\),则\({{s}_{1}}\)_________\({{s}_{2}}(\)填\(" > "\),\("="\)或\(" < ")\).
\((2)\)基于上述思考,小泽进行了如下探究:
\(①\)如图\(2\),点\(C\)在线段\(AB\)上,正方形\(FGBC\), \(ACDE\)和\(EDMN\),其面积比为\(1:4:4\),连接\(AF\),\(AM\),求证\(AF⊥AM\);
\(②\)如图\(3\),点\(C\)在线段\(AB\)上,点\(D\)是线段\(CF\)的黄金分割点,正方形\(ACDE\)和矩形\(CBGF\)的面积相等,连接\(AF\)交\(ED\)于点\(M\),连接\(BF\)交\(ED\)延长线于点\(N\),当\(CF=a\)时,直接写出线段\(MN\)的长为____________.