[填空题]
Let $ \mathbf{u}=\left(\begin{array}{c}1 \\ 3 \\ -2\end{array}\right)$ and $ \mathbf{v}=\left(\begin{array}{c}k \\ 2 \\ -4\end{array}\right)$ where k>0 . The angle between$ \mathbf{u}$ and $\mathbf{v} $ is $ \frac{\pi}{4}$ . Find the value of k .≈
