TeX Quellcode:

\underbrace{\frac{a}{b}}_{\frac{a}{b}\cdot \frac{d}{d}}+\underbrace{\frac{c}{d}}_{\frac{c}{d}\cdot \frac{b}{b}}=\frac{ad}{bd}+\frac{cb}{bd} = \frac{ad+cb}{bd}