Hello, I do not know if it's really upper physics but I have some exercices to do and I find them a bit difficult for a beginning. This is a circuit: http://up.sur-la-toile.com/iu7M I'm asked to simplify it like shown on the drawing so that afterwards I get this circuit: http://up.sur-la-toile.com/iu7L I can simplify it by following the instance but I just would like to know what formula was used to simplify the circuit. 2) Then I have to determine the relation between r and R; Va7, E and R then i'm supposed to deduce I7. This is what I've done (using Kirchhoff's laws): I7=Va7/2(R+r) is it correct ? However, I don't see the relation between r and R except maybe: E=(R+r)i.... Thank you for any valuable help!