Problem

With respect to a fixed origin $O$, the equations of lines $l_1$ and $l_2$ are given by

$$l_1:\ \mathbf{r}=\begin{pmatrix}2\\8\\10\end{pmatrix}+\lambda\begin{pmatrix}-1\\2\\3\end{pmatrix}$$ $$l_2:\ \mathbf{r}=\begin{pmatrix}-4\\-1\\2\end{pmatrix}+\mu\begin{pmatrix}5\\4\\8\end{pmatrix}$$

where $\lambda$ and $\mu$ are scalar parameters.

Prove that lines $l_1$ and $l_2$ are skew.