a)-1/21+-1/21 b)-8/18-15/27 c)-5/12+0,75 d)3,5-(-2/7

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 +$\frac{2}{7}$

= $\frac{7}{2}$ + $\frac{2}{7}$

= $\frac{53}{14}$