If V = 12R / (r + R) , then R =
\(V=\frac{12R}{r+R}\\\\V(r+R)=12R\\\\Vr+VR=12R\\\\VR-12R=-Vr\\\\(V-12)R=-Vr\ \ \ \ /:(V-12) eq0\\\\R=\frac{-Vr}{V-12}\\\\R=\frac{Vr}{12-V}\)
vr+vR=12Rvr=12R-vRvr=R(12-v)R=vr/12-v