If \( \cot\theta = -\dfrac{1}{2\sqrt{2}} \), where \( \theta \in \left( \dfrac{3\pi}{2}, 2\pi \right) \), then the value of
\[
\sin\left(\dfrac{15\theta}{2}\right)(\sin 8\theta + \cos 8\theta)
+ \cos\left(\dfrac{15\theta}{2}\right)(\cos 8\theta - \sin 8\theta)
\]
is: