Division des fractions algébriques: $\frac{\frac{a}{b}}{\frac{c}{d}} = \frac{a}{b} : \frac{c}{d} = \frac{ad}{bc}$

Got it !

 $\frac{a}{b} : \frac{b}{a} =$ $\frac{a}{b} \cdot \frac{a}{b} = \frac{a^2}{b^2}$ $\frac{a}{6b} : \frac{3b}{4} =$ $\frac{4a}{18b^2}$ $\frac{\frac{1}{b}}{\frac{b}{3}} =$ $\frac{3}{b^2}$