Bài 42 :

`a,[-7]/25+[-8]/[25]`

`=[(-7)+(-8)]/25`

`=[-15]/25`

`=-3/5`

$b,\dfrac{1}{6} + \dfrac{-5}{6}\\ = \dfrac{1 + (-5)}{6}\\ = \dfrac{-4}{6}\\ = \dfrac{-2}{3}$

$c,\dfrac{6}{13} + \dfrac{-14}{39}\\ = \dfrac{18}{39} + \dfrac{-14}{39}\\ = \dfrac{18 + (-14)}{39}\\ = \dfrac{4}{39}$

$d, \,\, \dfrac{4}{5} + \dfrac{4}{-18} \\ = \dfrac{4}{5} + \dfrac{-2}{9} \\ = \dfrac{36}{45}+\dfrac{-10}{45} \\ = \dfrac{36 + (-10)}{45} \\ = \dfrac{26}{45}$

Bài 43 :

$a,\dfrac{7}{21} + \dfrac{9}{-36}\\ = \dfrac{1}{3} + \dfrac{-1}{4} \\= \dfrac{4}{12} + \dfrac{-3}{12}\\ = \dfrac{1}{12}$

$b,\dfrac{-12}{18} + \dfrac{-21}{35} \\= \dfrac{-2}{3} + \dfrac{-3}{5}\\ = \dfrac{-10}{15} + \dfrac{-9}{15}\\ = \dfrac{-19}{15}$

$c,\dfrac{-3}{21} + \dfrac{6}{42} \\= \dfrac{-1}{7} + \dfrac{1}{7} \\= 0$

$d,\dfrac{-18}{24} + \dfrac{15}{-21}\\ = \dfrac{-3}{4} + \dfrac{-5}{7}\\ = \dfrac{-21}{28} + \dfrac{-20}{28}\\ = \dfrac{-41}{28}$

Hình ảnh đầu là bài 42

Hình ảnh cuỗi là bài 43