That's what Im not sure about... That's all the question say and I thought maybe that was a notation for a formula I'm totally missing.
I believe that a b and c are side of a triangle and r is a radius, so I'm guessing R is also a radius.
s i'm not sure about, its either a radius or a semi perimeter. There is no picture or explanation of any kind
here (scroll down to the section "Scalene Triangle"), but without proof.