(c)
Let $O$ is orthocenter, $G$ is centroid and $C$ is circumcentre, then
[image:5_image6.emf]
$\ \frac{\mathcal{z}_{1} + \mathcal{z}_{2} + \mathcal{z}_{3}}{3} = \frac{2 \times 0 + 1(\mathcal{z)}}{3}$
$\mathcal{\Rightarrow \ \ z =}\mathcal{z}_{1} + \mathcal{z}_{2} + \mathcal{z}_{3}$