In a triangle ABC, if r1 is the exradius of the excircle opposite to A, r2 is the exradius of the excircle opposite to B and r3 is the exradius of the excircle opposite to C....
prove that (ar1+br2+cr3)/abc=(1/r)-(1/2R)
where r is the inradius and R is the circum radius.
formulas
r1=(area of triangle)/(s-a)
r1=stanA/2