Length of opposite side from central angle and radius

I found this, a is the central angle and r is the radius of the circle, the line BC's length is simply 2*r*sin(a), I just thought that was interesting because I don't remember seeing that before... :

**I think the neat thing that follows is for some triangle with sides A,B, C, there is something the sum of the sines of the 3 opposing angles have to add up to, looking at the circumcenter of that triangle inscribed in a circle, or maybe even it extends to any polytope inscribed in a circle, but I'll have to work on that later...