3.
已知\( \overrightarrow{a}=(\sin x,-\cos x), \overrightarrow{b}=( \sqrt {3}\cos x,-\cos x),f(x)=2 \overrightarrow{a}\cdot \overrightarrow{b}\)
\((1)\)求的\(f(x)\)解析式;
\((2)\)在\(\triangle ABC\)中,\(a\),\(b\),\(c\)分别是内角\(A\),\(B\),\(C\)的对边,若\(f(A)=2\),\(b=1\),\(\triangle ABC\)的面积为\( \dfrac { \sqrt {3}}{2}\),求\(a\)的值.