38. The center of the sphere is located in the plane of the base of a regular triangular pyramid. The vertices of the base lie on the surface of the sphere. Find $l$ - the length of the line of intersection of the surfaces of the sphere and the pyramid, if the radius of the sphere is $R$, and the plane angle at the vertex of the pyramid is $\alpha$.
38. If $0<\alpha<\arccos \frac{1}{4}$,

$$
l=R \sqrt{27+3 \operatorname{tg}^{2} \frac{\alpha}{2}}\left[\operatorname{arctg}\left(3 \operatorname{ctg} \frac{\alpha}{2}\right)-\alpha\right] ;
$$

if $\alpha \geqslant \arccos \frac{1}{4}, l=0$.