Never mind the not connected branches a and b in the middle. They are part of another exercise which isn't related to my question.
Lets say I want to calculate the current in the branch that contains AB. (I know its Isrc/2 but I want to calculate it using the formula.)
Iab = Req Isrc / Rab
Req is the equivalent resistance of the two branches in parallel.
Req= (R1+R2 ) /2
My question is when is Req the equivalent of Rab and Rcd only ? When the branch ab (the ones I told you to forget) is connected for example? And why is that ?