1. Given the radius of a circle is $r$, the side length of its inscribed regular pentagon is $a_{5}=r \sqrt{\frac{5-\sqrt{5}}{2}}$, then the side length of the inscribed regular decagon of the circle $a_{10}=$ $\qquad$
$=1 \cdot \frac{1}{2}(\sqrt{5}-1) r$