- $$\frac{{\left( {{{\text{r}}^2} + {{\text{R}}^2}} \right)\cos \varphi }}{2}$$
- $$\frac{{\left( {{{\text{r}}^2} + {{\text{R}}^2}} \right)\sin \varphi }}{2}$$
- $$\frac{{\left( {{\text{r}} + {\text{R}}} \right)\cos \varphi }}{2}$$
- $$\frac{{\left( {{\text{r}} + {\text{R}}} \right)\sin \varphi }}{2}$$