已知\({\tan }\theta =-\dfrac{3}{4}\),求下列各式的值:
\((1)\dfrac{{\sin }\left( \theta +\dfrac{3\pi }{2} \right)+{\cos }\left( \theta -\dfrac{\pi }{2} \right)}{2{\sin }\left( \pi +\theta \right)+{\cos }\left( \theta -\pi \right)}\)
\((2){\sin }\theta {\cos }\theta -{co}{{{s}}^{2}}\theta \)