- 
                                   
                                   
                                    A 2 R1 + R2/R2 - R1.
- 
                                   
                                   
                                    B 2 R2 - R1/R1R2.
- 
                                   
                                   
                                    C 2 R1 + R2/R2R1.
- 
                                   
                                   
                                    D 2 R2 - R1/R1 + R2.
- 
                                   
                                   
                                    E 2 R1R2/R2 - R1.
