12.3. Express the area of triangle $ABC$ in terms of the length of side $BC$ and the measures of angles $B$ and $C$.
12.3. By the Law of Sines $b=a \sin \beta / \sin \alpha=a \sin \beta / \sin (\beta+\gamma)$, therefore $S=$ $=a b \sin \gamma / 2=a^{2} \sin \beta \sin \gamma / 2 \sin (\beta+\gamma)$.