(b)
We have,$\ \ \frac{x}{\cos\theta} = \frac{y}{\cos\left( \theta - \frac{2\pi}{3} \right)} = \frac{\mathcal{z}}{\cos\left( \theta + \frac{2\pi}{3} \right)}$
Therefore, each ratio is equal to$\ \ \frac{x + y + \mathcal{z}}{\cos{\theta + \cos{\left( \theta - \frac{2\pi}{3} \right) + \cos\left( \theta + \frac{2\pi}{3} \right)}}}$
$= \frac{x + y + \mathcal{z}}{\cos{\theta + 2\cos{\theta\cos\frac{2\pi}{3}}}}$
$= \frac{x + y + \mathcal{z}}{0}$
$\Rightarrow x + y + \mathcal{z} = 0$