6.
在正方形\(ABCD\)中,点\(E{,}F\)分别在边\({BC}{,}{CD}\)上,且\({∠}{EAF}{=∠}{CEF}{=}45^{{∘}}\).
\((1)\)将\({\triangle }{ADF}\)绕着点\(A\)顺时针旋转\(90^{{∘}}\),得到\({\triangle }{ABG}(\)如图\({①})\),求证:\({\triangle }{AEG}\)≌\({\triangle }{AEF}\);
\((2)\)若直线\(EF\)与\({AB}{,}{AD}\)的延长线分别交于点\(M{,}N(\)如图\({②})\),求证:\(EF^{2}{=}ME^{2}{+}NF^{2}\);
\((3)\)将正方形改为长与宽不相等的矩形,若其余条件不变\((\)如图\({③})\),请你直接写出线段\({EF}{,}{BE}{,}{DF}\)之间的数量关系.