9.
已知向量 \( \overrightarrow{a}=(\cos α,\sin α)\),\( \overrightarrow{b}=(\cos β,\sin β)\),\(| \overrightarrow{a}- \overrightarrow{b}|=1\).
\((1)\)求\(\cos (α-β)\)的值;
\((2)\)若\(- \dfrac {π}{2} < β < 0 < α < \dfrac {π}{2}\),且\(\sin β=- \dfrac {1}{7}\),求\(\sin α\)的值.