Rr_3ers_innen = Rr_3 + R_innen
Rr_ers23_innen = 1/sum(1 ./ [Rr_2 Rr_3ers_innen])
Ir_ges_innen = U2 / (Rr_ers23_innen + Rr_1);
Ur_23_innen = Rr_ers23_innen * Ir_ges_innen
Ir_2_innen = Ur_23_innen / Rr_2
Ir_3_innen = Ur_23_innen / Rr_3
Ur_innen = Ir_3_innen * R_innen