When two resistors $S$ and $T$ are connected in parallel their combined resistance $R$ is given by the expression $$R=\frac{1}{\frac{1}{S}+\frac{1}{T}}.$$ This expression can be rewritten as a rational expression in $S$ and $T$. The numerator of that expression is and its denominator is
Hint: Rational expressions work exactly like fractions.