Question 113426
Your equation is unclear.
Do you mean 
{{{A=1/(2(b+B))}}}
or do you mean
{{{A=(1/2)*(b+B))}}}
We'll do it both ways. 
{{{A=(1/2)*(b+B))}}}
{{{2A=b+B }}}  Multiply both sides by 2.
{{{2A-b=B}}} Subtract b from boths sides. 
{{{B=2A-b}}}
and 
{{{A=1/(2(b+B))}}}
{{{2A(b+B)=1)}}} Multiply both sides by 2(b+B).
{{{2Ab+2AB=1}}} Distribute 2A through.
{{{2AB=1-2Ab}}} Subtract 2Ab from both sides.
{{{B=(1-2Ab)/(2A)}}} Divide both sides by 2A.