$(a)\;(\phi_{2}-\phi_{1})\;\in_{0}\qquad(b)\;(\phi_{1}+\phi_{2})\;\in_{0}\qquad(c)\;\large\frac{(\phi_{2}-\phi_{1})}{ \in_{0}}\qquad(d)\;\large\frac{(\phi_{1}+\phi_{2})}{\in_{0}}$