Question 351557
{{{1/r=1/r1+1/r2}}}
Multiply both sides by {{{r*r1*r2}}}, assuming {{{r<>0}}},{{{r1<>0}}},{{{r2<>0}}}
{{{r1*r2=r*r2+r*r1}}}
{{{r(r1+r2)=r1*r2}}}
{{{highlight_green(r=(r1*r2)/(r1+r2))}}}