\((1)\)【发现】
如图\(①\),\(∠\)\(ACB\)\(=∠\)\(ADB\)\(=90^{\circ}\),那么点\(D\) 经过\(A\),\(B\),\(C\)三点的圆上\((\)填“在”或“不在”\()\).
\((2)\)【思考】
如图\(②\),如果\(∠\)\(ACB\)\(=∠\)\(ADB\)\(=\)\((\)\(\neq 90^{\circ})(\)点\(C\),\(D\)在\(AB\)的同侧\()\),那么点\(D\)还在经过\(A\),\(B\),\(C\)三点的圆上吗?
请证明点\(D\)也不在\(⊙\)\(O\)内\(.\)
\((3)\)【应用】
利用【发现】和【思考】中的结论解决问题:
若四边形\(ABCD\)中,\(AD\)\(/\!/\)\(BC\),\(∠\)\(CAD\)\(=90^{\circ}\),点\(E\)在边\(AB\)上,\(CE\)\(⊥\)\(DE\).
\(①\)作\(∠\)\(ADF\)\(=∠\)\(AED\),交\(CA\)的延长线于点\(F\)\((\)如图\(④)\),求证:\(DF\)为\(Rt\)\(\triangle \)\(ACD\)的外接圆的切线;
\(②\)如图\(⑤\),点\(G\)在\(BC\)的延长线上,\(∠\)\(BGE\)\(=∠\)\(BAC\),已知\(\sin \angle AED=\dfrac{2}{3}\),\(AD\)\(=1\),求\(DG\)的长.