8.
\(\triangle ABC\)中,\(a=4\),\(b=5\),\(C= \dfrac {2π}{3}\),角\(A\)、\(B\)、\(C\)所对的边分别为\(a\)、\(b\)、\(c\),点\(D\)在边\(AB\)上,且\( \dfrac {AD}{DB}= \dfrac {2}{3}\).
\((1)\)用\( \overrightarrow{CA}\)和\( \overrightarrow{CB}\)表示\( \overrightarrow{CD}\);
\((2)\)求\(|CD|\).