已知函数\(f\left(x\right)=2\sin \left(ωx+φ\right)\left(ω > 0,- \dfrac{π}{2}φ < \dfrac{π}{2}\right) \)的部分图象如图所示,直线\(x=\dfrac{\pi }{12}\),\(x=\dfrac{7\pi }{12}\)是其相邻的两条对称轴.
\((1)\)求函数\(f(x)\)的解析式;
\((2)\)若\(f(\dfrac{a}{2})=-\dfrac{6}{5}\),且\( \dfrac{2π}{3} < α < \dfrac{7π}{6} \),求\(\cos α\)的值.