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