已知函数 $f\left(x\right)=2\sin (\frac{1}{3}x-\frac{\pi }{6}),x\in R$.

(1)求 $f\left(\frac{5\pi }{4}\right)$的值;

(2)设 $\alpha ,\beta \in [0,\frac{\pi }{2}],f(3\alpha +\frac{\pi }{2})=\frac{10}{13},f(3\beta +2\pi )=\frac{6}{5}$，求 $\cos (\alpha +\beta )$的值.