Đáp án:
Giải thích các bước giải:
a)
=$\dfrac{-12}{18}$ -- $\dfrac{21}{35}$
=$\dfrac{-12}{18}$ + $\dfrac{21}{35}$
=$\dfrac{-2}{3}$+ $\dfrac{3}{5}$
=$\dfrac{-10}{15}$+$\dfrac{9}{15}$
=$\dfrac{-1}{15}$
b)
$\dfrac{-3}{21}$-$\dfrac{6}{42}$
=$\dfrac{-1}{7}$-$\dfrac{3}{5}$
=$\dfrac{-5}{35}$-$\dfrac{21}{35}$
==$\dfrac{-26}{35}$
c)
$\dfrac{-18}{24}$-$\dfrac{15}{21}$
=$\dfrac{-3}{4}$-$\dfrac{5}{7}$
=$\dfrac{-21}{28}$-$\dfrac{20}{28}$
=$\dfrac{-41}{28}$
