Bài 12:

Phân số tối giản là:

$\frac{2}{3}$ ; $\frac{-13}{-12}$ ; $\frac{-21}{-17}$ 

Bài 13:

a) $\frac{4}{12}$ = $\frac{4 : 4}{12:4}$ = $\frac{1}{3}$ 

b) $\frac{15}{60}$ = $\frac{15:15}{60:15}$ = $\frac{1}{4}$ 

c) $\frac{-4}{12}$ = $\frac{14:4}{12:4}$ = $\frac{-1}{3}$ 

d) $\frac{-14}{-28}$ = $\frac{19:19}{-28:14}$ = $\frac{-1}{-2}$ = $\frac{1}{2}$

e) $\frac{35}{140}$ = $\frac{35:35}{140:35}$ = $\frac{1}{4}$

f) $\frac{-15}{-25}$ = $\frac{-15:(-5)}{-25:(-5)}$ = $\frac{3}{5}$

g) $\frac{-16}{-24}$ = $\frac{-16:(-8) }{-24:(-8)}$ = $\frac{2}{3}$

h) $\frac{42}{-28}$ = $\frac{42:(-19)}{-28 :(-19)}$ = $\frac{-3}{2}$

Bài 14:

a) $\frac{55}{66}$ = $\frac{5.11}{6.11}$ = $\frac{5}{6}$ 

b) $\frac{-33}{88}$ = $\frac{-3.11}{8.11}$ = - $\frac{3}{8}$ 

c) $\frac{2020}{8080}$ = $\frac{2020}{4.2020}$ = $\frac{1}{4}$ 

d) $\frac{-2121}{-4242}$ = $\frac{-1.2121}{-2.2121}$ = $\frac{1}{2}$ 

e) $\frac{130130}{-260260}$ = $\frac{130130}{-2.130130}$ = $\frac{-1}{2}$ 

f) $\frac{1616}{-1313}$ = $\frac{101.16}{-101.13}$ = $\frac{-16}{13}$

Hù...Mệt quá!!! Mik làm đến đây thôi... :<