a) $\frac{32}{12}$ = $\frac{32:4}{12:4}$ = $\frac{8}{3}$
$\frac{15.3-15}{3-18}$ = $\frac{15.(3-1)}{-15}$ = $\frac{15.2}{-15}$ = $\frac{-1.2}{1}$ = -2
b) $\frac{2}{3}$ = $\frac{2.4}{3.4}$ = $\frac{8}{12}$
$\frac{3}{4}$ = $\frac{3.3}{4.3}$ = $\frac{9}{12}$
Vì $\frac{8}{12}$ < $\frac{9}{12}$ nên $\frac{2}{3}$ < $\frac{3}{4}$