Question 321568
{{{1/r = 1/R1 + 1/R2}}}
 Multiply both sides by {{{r*R1*R2}}}
{{{R1*R2=r*R2+r*R1}}}
{{{R1*R2-r*R1=r*R2}}}
{{{R1*(R2-r)=r*R2}}}
{{{R1=(r*R2)/(R2-r)}}}