Question 88354
{{{I=E/(R+r)}}}

Since {{{a/b=c/d}}} means that {{{ad=bc}}}, it follows that
{{{I/1=E/(R+r)}}} means that {{{I(R+r) = E*1}}}


Remove parentheses:
{{{IR+ Ir= E}}}


Get the R term alone on the left side by subtracting Ir from each side:
{{{IR = E-Ir}}}


Divide both sides by I in order to undo the multiplication of I times R:
{{{(IR)/I= (E-Ir)/I}}}
{{{R = (E-Ir)/I}}}


R^2