7.
在\(\Delta ABC\)中,角\(A,B,C\)的对边分别为\(a,b,c,\cos C=\dfrac{3}{10}\).
\((1)\)若\(\overrightarrow{CA}\bullet \overrightarrow{CB}=\dfrac{9}{2}\),求\(\Delta ABC\)的面积;
\((2)\)设向量\( \overset{⇀}{x}=(2\sin B,− \sqrt{3}), \overset{⇀}{y}=(\cos 2B,1−2{\sin }^{2} \dfrac{B}{2}) \),且\( \overset{⇀}{x}/\!/ \overset{⇀}{y} \),求角\(B\)的值.