如图,在\(Rt\triangle ABC\)中,\(∠ACB=90^{\circ}\),\( \dfrac {BC}{AC}= \dfrac {m}{n}\),\(CD⊥AB\)于点\(D\),点\(E\)是直线\(AC\)上一动点,连接\(DE\),过点\(D\)作\(FD⊥ED\),交直线\(BC\)于点\(F\).
\((1)\)探究发现:
如图\(1\),若\(m=n\),点\(E\)在线段\(AC\)上,则\( \dfrac {DE}{DF}=\) ______ ;
\((2)\)数学思考:
\(①\)如图\(2\),若点\(E\)在线段\(AC\)上,则\( \dfrac {DE}{DF}=\) ______ \((\)用含\(m\),\(n\)的代数式表示\()\);
\(②\)当点\(E\)在直线\(AC\)上运动时,\(①\)中的结论是否任然成立?请仅就图\(3\)的情形给出证明;
\((3)\)拓展应用:若\(AC= \sqrt {5}\),\(BC=2 \sqrt {5}\),\(DF=4 \sqrt {2}\),请直接写出\(CE\)的长.