Đáp án+Giải thích các bước giải:
$\\C)\dfrac{-8}{18}+\dfrac{-15}{27}=\dfrac{-4}{9}+\dfrac{-5}{9}=\dfrac{-9}{9}=-1$ $\\D)\dfrac{7}{12}+\dfrac{9}{-36}=\dfrac{7}{12}+\dfrac{-3}{12}=\dfrac{4}{12}=\dfrac{1}{3}$ $\\E)\dfrac{-12}{18}+\dfrac{-21}{35}=\dfrac{-2}{3}+\dfrac{-3}{5}=\dfrac{-10}{15}+\dfrac{-9}{15}=\dfrac{-19}{15}$ $\\F)(-2)+\dfrac{-5}{8}=\dfrac{-16}{8}+\dfrac{-5}{8}=\dfrac{-21}{8}$