6.
\((\)Ⅰ\()\;①\) 证明两角和的余弦公式\({C}_{α+β}:\cos (α+β)=\cos α\cos β-\sin α\sin β \);
\(\;②\) 证明:\(\sin 3\alpha =3{{\sin }^{2}}\alpha -4{{\sin }^{3}}\alpha \).
\((\)Ⅱ\()\) 已知\(\triangle ABC\)的面积\(S= \dfrac{1}{2}, \overrightarrow{AB}· \overrightarrow{AC}=3 \), 且\(\cos B= \dfrac{3}{5} \), 求\(\cos C\).