Ta có
$C = \dfrac{\sin(x+y)}{\sin x + \sin y}$
$= \dfrac{2\sin\left( \frac{x + y}{2} \right) \cos \left( \frac{x + y}{2} \right)}{2 \sin \left( \frac{x + y}{2} \right) \cos \left( \frac{x-y}{2} \right)}$
$= \dfrac{\cos \left( \frac{x + y}{2} \right)}{\cos \left( \frac{x-y}{2} \right)}$
Vậy
$C= \dfrac{\cos \left( \frac{x + y}{2} \right)}{\cos \left( \frac{x-y}{2} \right)}$
