Answer :

[tex]V=\frac{12R}{r+R}\\\\V(r+R)=12R\\\\Vr+VR=12R\\\\VR-12R=-Vr\\\\(V-12)R=-Vr\ \ \ \ /:(V-12)\neq0\\\\R=\frac{-Vr}{V-12}\\\\R=\frac{Vr}{12-V} [/tex]
vr+vR=12R
vr=12R-vR
vr=R(12-v)
R=vr/12-v

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