Đáp án:
Giải thích các bước giải:
a)
$\dfrac{1}{2} + \dfrac{-1}{5}$
= $\dfrac{5}{10} + \dfrac{-2}{10}$
= $\dfrac{3}{10}$
b)
$\dfrac{2}{-7} - \dfrac{3}{4}$
$=$ $\dfrac{-2}{7} - \dfrac{3}{4}$
$=$ $\dfrac{-8}{28} - \dfrac{21}{28}$
$=$ $\dfrac{-29}{28}$
c)
$\dfrac{-1}{2} . \dfrac{1}{3} + \dfrac{-1}{2} . \dfrac{2}{3}$
$=$ $\dfrac{-1}{2} . (\dfrac{1}{3} + \dfrac{2}{3})$
$=$ $\dfrac{-1}{2} . 1$
$=$ $\dfrac{-1}{2}$