1.
已知向量\( \overrightarrow{a}=(\sin x, \dfrac {3}{2})\),\( \overrightarrow{b}=(\cos x,-1)\),
\((\)Ⅰ\()\)当\( \overrightarrow{a}/\!/ \overrightarrow{b}\)时,求\(\tan 2x\)的值;
\((\)Ⅱ\()\)求函数\(f(x)=( \overrightarrow{a}+ \overrightarrow{b})⋅ \overrightarrow{b}\)在\([- \dfrac {π}{2},0]\)上的值域.