Question 10329
Do you mean {{{I = E/(R + r)}}},  solve for r??  If this is what you meant, then you needed parentheses around the (R+r).  If that is NOT what you meant, then the solution is quite different from what I am about to do--resubmit the problem, we'll take another look at it.   Also, remember that
in math, capital R and lower case r are considered two different variables!


Assuming this is the problem you had in mind:
{{{I/1 = E/(R+r) }}}.  Remember {{{a/b=c/d}}}?  It means {{{ad= bc}}}.


In the same way, {{{I/1 = E/(R+r) }}} means that 
{{{I*(R+r) = 1*E}}}


Remove parentheses:
IR + Ir = E


Get the small r term all alone on the left side by subtracting IR from each side:
IR - IR + Ir = E - IR
Ir = E-IR


Last, divide both sides by I:
{{{ (Ir)/I = (E-IR)/I }}}
{{{ r = (E-IR)/I }}}


R^2 at SCC