Question 300921
{{{(1)/(a)+(1)/(b)= (1)/(c)}}} 
Multiply by {{{abc}}}.
{{{bc+ac=ab}}}
Move "a" terms on one side, everything else on the other.
{{{ac-ab=-bc}}}
Factor out the "a".
{{{a(c-b)=-bc}}}
{{{a=-bc/(c-b)}}}
{{{a=bc/(b-c)}}}
Assumption is that {{{b-c}}} doesn't equal zero.