已知函数\(f(x)= \sqrt{3}\sin \dfrac{x}{2}\cos \dfrac{x}{2}-{\cos }^{2} \dfrac{2x}{2}+ \dfrac{1}{2} \).
\((1)\)求函数\(f(x)\)的单调递减区间;
\((2)\)若\(\triangle ABC\)的内角\(A\),\(B\),\(C\)所对的边分别为\(a\),\(b\),\(c\),\(f(A)= \dfrac{1}{2},a= \sqrt{3} \),\(\sin B=2\sin C\),求\(c\).