Cách làm cho bạn:
a) \(\frac{7}{21}+\frac{9}{-36}=\frac{1}{3}+\frac{-1}{4}=\frac{4}{12}+\frac{-3}{12}=\frac{1}{12}\)
b) \(\frac{-12}{18}+\frac{-21}{35}=\frac{-2}{3}+\frac{-3}{5}=\frac{-10}{15}+\frac{-9}{15}=\frac{-19}{15}\)
c) \(\frac{-3}{21}+\frac{6}{42}=\frac{-1}{7}+\frac{1}{7}=0\)
d) \(\frac{-18}{24}+\frac{15}{-21}=\frac{-3}{4}+\frac{-5}{7}=\frac{-21}{28}+\frac{-20}{28}=\frac{-41}{28}\)
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