如图所示,空间四边形\(OABC\)中,\(\overrightarrow{OA}=a\),\(\overrightarrow{OB}=b\),\(\overrightarrow{OC}=c\),点\(M\)在\(OA\)上,且\(OM=2MA\),点\(N\)为\(BC\)的中点,则\(\overrightarrow{MN}\)等于\((\) \()\)
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