1.
a)$\cfrac{12}{15}$ = $\cfrac{16}{20}$ ; $\cfrac{20}{15}$ = $\cfrac{16}{12}$ ; $\cfrac{20}{16}$ = $\cfrac{15}{12}$ ; $\cfrac{12}{16}$ = $\cfrac{15}{20}$
b)$\cfrac{2,4}{8}$ = $\cfrac{0,96}{3,2}$ ; $\cfrac{3,2}{8}$ = $\cfrac{0,96}{2,4}$ ; $\cfrac{2,4}{0,96}$ = $\cfrac{8}{3,2}$ ; $\cfrac{3,2}{0,96}$ = $\cfrac{8}{2,4}$
c)$\cfrac{-1,2}{3,6}$ = $\cfrac{-5}{15}$ ; $\cfrac{15}{3,6}$ = $\cfrac{-5}{1,2}$ ; $\cfrac{-1,2}{-5}$ = $\cfrac{3,6}{15}$ ; $\cfrac{15}{-5}$ = $\cfrac{3,6}{-1,2}$
d)$\cfrac{243}{81}$ = $\cfrac{27}{9}$ ; $\cfrac{9}{81}$ = $\cfrac{27}{243}$ ; $\cfrac{243}{27}$ = $\cfrac{81}{9}$ ; $\cfrac{9}{27}$ = $\cfrac{81}{243}$
2. Tất cả các tỉ lệ thức:
$\cfrac{1}{5}$ = $\cfrac{125}{625}$ ; $\cfrac{625}{5}$ = $\cfrac{125}{1}$ ; $\cfrac{1}{125}$ = $\cfrac{5}{625}$ ; $\cfrac{625}{125}$ = $\cfrac{5}{1}$