II. Tự luận

- Hướng dẫn giải

a. $\frac{-3}{6}+\frac{3}{12}+\frac{-3}{4}=\frac{-6}{12}+\frac{3}{12}+\frac{-9}{12}$$=\frac{-6+3-9}{12}=\frac{-12}{12}=-1$

b. $\frac{3}{7}+\left(\frac{-1}{5}+\frac{-3}{7}\right)=\left(\frac{3}{7}+\frac{-3}{7}\right)+\frac{-1}{5}=0+\frac{-1}{5}=\frac{-1}{5}$

c. $\left(\frac{-2}{8}+\frac{5}{9}+\frac{1}{2015}\right)+\frac{1}{4}+\frac{-5}{9}$

$$\begin{gathered} =\left(\frac{-2}{8}+\frac{1}{4}\right)+\left(\frac{5}{9}+\frac{-5}{9}\right)+\frac{1}{2015} \\ =0+0+\frac{1}{2015} \\ =\frac{1}{2015} \end{gathered}$$