5.
已知向量\( \overrightarrow{a}=(\sin x,-1)\),\( \overrightarrow{b}=( \sqrt {3}\cos x,- \dfrac {1}{2}).\)函数\(f(x)=( \overrightarrow{a}+ \overrightarrow{b})⋅ \overrightarrow{a}-2\).
\((1)\)求函数\(f(x)\)的单调递减区间;
\((2)\)已知\(a\),\(b\),\(c\)分别为\(\triangle ABC\)内角\(A\),\(B\),\(C\) 的对边,其中\(A\)为锐角,\(a=2 \sqrt {3}\),\(c=4\),且\(f(A)=1\),求\(\triangle ABC\)的面积.