11.
$\frac{1}{4}$=$\frac{3}{12}$; $\frac{1}{4}$=$\frac{2}{8}$
$\frac{-3}{4}$=$\frac{-6}{8}$; ... (Có nhiều kết quả khác nhau);
$1$=$\frac{2}{2}$=$\frac{-4}{-4}$=$\frac{6}{6}$=$\frac{-8}{-8}$=$\frac{10}{10}$
12.
a)$\frac{-3}{6}$=$\frac{-1}{2}$;
b)$\frac{2}{7}$=$\frac{8}{28}$;
c)$\frac{-15}{25}$=$\frac{-3}{5}$;
d)$\frac{4}{9}$=$\frac{4.7}{9.7}$=$\frac{28}{63}$
13.
a)$\frac{15}{60}$=$\frac{15:15}{60:15}$=$\frac{1}{4}$ giờ
b)$\frac{30}{60}$=$\frac{30:30}{60:30}$=$\frac{1}{2}$ giờ
c)$\frac{45}{60}$=$\frac{45:15}{60:15}$=$\frac{3}{4}$ giờ
d)$\frac{20}{60}$=$\frac{20:20}{60:20}$=$\frac{1}{3}$ giờ
e)$\frac{40}{60}$=$\frac{40:20}{60:20}$=$\frac{2}{3}$ giờ
g)$\frac{10}{60}$=$\frac{10:10}{60:10}$=$\frac{1}{5}$ giờ
h)$\frac{5}{60}$=$\frac{5:5}{60:5}$=$\frac{1}{12}$ giờ
