9.
已知函数\(f\left(x\right)=\begin{vmatrix} \sqrt{3}{\cos }^{2}x & -\sin x \\ \cos x & 1\end{vmatrix} \);
\((1)\)当\(x∈\left[0, \dfrac{π}{2}\right] \)时,求\(f\left(x\right) \)的值域;
\((2)\)已知\(\triangle ABC \)的内角\(A,B,C \)的对边分别为\(a,b,c \),若\(f\left( \dfrac{A}{2}\right)= \sqrt{3} \),\(a=4 \),\(b+c=5 \),求\(\triangle ABC \)的面积;