7, A moving point starts from the origin and first moves along the $x$-axis. After completing each unit length, it turns left by an angle $\theta$. Find the distance of this moving point from the origin after it has traveled $n$ unit lengths, which equals


$\begin{array}{l}7 . \overline{O P_{n} \mid} \\ =\left|\frac{\sin \frac{n}{2} \theta}{\sin \frac{1}{2} \theta}\right|\end{array}$