Find the exact value of the expression:
The expression is undefined.
$\cos\bigg($\frac{5\pi }{6}$\bigg)\cos\bigg($\frac{2\pi}{3}$\bigg)+\sin\bigg($\frac{5\pi}{6}$\bigg)\sin\bigg($\frac{2\pi}{3}$\bigg)$
