Bạn tham khảo :
a) $\frac{-1}{21}$ + $\frac{-1}{21}$
= $\frac{-2}{21}$
b) $\frac{-8}{18}$ - $\frac{15}{27}$
=$\frac{24}{54}$ - $\frac{30}{54}$
= -1
c) $\frac{-5}{12}$ + 0.75
= $\frac{-5}{12}$ + $\frac{3}{4}$
= $\frac{-5}{12}$ + $\frac{9}{12}$
= $\frac{1}{3}$
d) 3,5 -(-$\frac{2}{7}$)
= 3,5 +$\frac{2}{7}$
= $\frac{7}{2}$ + $\frac{2}{7}$
= $\frac{53}{14}$