fraction(S1 * N1, D1) + fraction(S2 * N2, D2) = {?}
\qquad =
-
\blue{\dfrac{N1 \times F1}{D1 \times F1}}
-+
\green{\dfrac{N2 \times F2}{D2 \times F2}}
\qquad =
-
\blue{\dfrac{N1 * F1}{LCM}}
-+
\green{\dfrac{N2 * F2}{LCM}}
\qquad =
\dfrac{
-
\blue{N1 * F1}
-+
\green{N2 * F2}
}{LCM}
\qquad = fraction(S1 * F1 * N1 + S2 * F2 * N2, LCM)
\qquad = fractionReduce(S1 * F1 * N1 + S2 * F2 * N2, LCM)