V=[tex] \frac{12R}{r+R} [/tex]
note, r and R are differnt
so multipl both sides by (r+R)
V(r+R)=12R
distribute
Vr+VR=12R
minus VR from both sides
Vr=12R-VR
undistribute R
Vr=R(12-V)
divide both sides by (12-V)
[tex] \frac{Vr}{12-V}=R [/tex] or
[tex] R= \frac{Vr}{12-V} [/tex]
