Inradius: r = Delta/s = (s-a)tan(A/2) = 4R sin(A/2)sin(B/2)sin(C/2). Exradius opposite A: r1 = Delta/(s-a) = stan(A/2) = 4R sin(A/2)cos(B/2)cos(C/2). Key relationship: 1/r = 1/r1 + 1/r2 + 1/r3. Also: r1r2r3 = rs^2 and r1 + r2 + r3 = r + 4R.