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已知\(\triangle OAB\)中,点\(D\)在线段\(OB\)上,且\(OD=2DB\),延长\(BA\)到\(C\),使\(BA=AC.\)设\( \overrightarrow{OA}= \overrightarrow{a}, \overrightarrow{OB}= \overrightarrow{b}\).
\((1)\)用\( \overrightarrow{a}, \overrightarrow{b}\)表示向量\( \overrightarrow{OC}, \overrightarrow{DC}\);
\((2)\)若向量\( \overrightarrow{OC}\)与\( \overrightarrow{OA}+k \overrightarrow{DC}\)共线,求\(k\)的值.