The voltage of the source V S is a known given value, but V D is on both sides of the equation, which forces an iterative solution: a starting value for V D is guessed and put into the right side of the equation. Carrying out the various operations on the right side, we come up with a new value for V D . This new value now is substituted on the right side, and so forth. If this iteration converges the values of V D become closer and closer together as the process continues, and we can stop iteration when the accuracy is sufficient. Once V D is found, I can be found from the Kirchhoff's law equation.