Question 196446
If you started with:
{{{Z=(Rr)/(R+r)}}}
Multiply both sides by (R+r):
{{{Z(R+r)=Rr}}}
{{{ZR+Zr=Rr}}}
{{{Zr=Rr-ZR}}}
{{{Zr=R(r-Z)}}}
{{{(Zr)/(r-Z)=R}}}