TeX source:
\frac{(a+b)}{(x+y)}-\frac{c}{d} = \frac{(a+b)}{(x+y)} \, \cdot\frac{d}{d} - \frac{c}{d} \, \cdot\frac{(x+y)}{(x+y)} = \frac{d(a+b)-c(x+y)}{d(x+y)}