These are ABCD network analysis equations. Any electrical network can be analysed using 4 different parameters
1) Z parameters (impedance parameters)
2) Y parameters (admnittance parameters)
3) H parameters (hybrid parameters)
4) ABCD parameters
This analysis is done to find out the i/p and o/p impedances and their inter-relationships. Also these are used to simplify and complex network into simple 2-port network.