Tính một cách hợp lí: a) (-5/6) - (-1,8) + (-1/6) - 0,8 b) (-9/7) + (-1,23) - (-2/7) - 0,77

* Hướng dẫn giải

$$\begin{gathered} a) \left(-\frac{5}{6}\right)-(-1,8)+\left(-\frac{1}{6}\right)-0,8 \\ =\left(-\frac{5}{6}\right)+\left(-\frac{1}{6}\right)-(-1,8)-0,8 \\ =-\left(\frac{5}{6}+\frac{1}{6}\right)+(1,8-0,8)= (- 1) + 1 = 0. \\ b) \left(-\frac{{\displaystyle 9}}{{\displaystyle 7}}\right)+(-1,23)-\left(-\frac{{\displaystyle 2}}{{\displaystyle 7}}\right)-0,77 \\ =\left(-\frac{{\displaystyle 9}}{{\displaystyle 7}}\right)-1,23+\frac{{\displaystyle 2}}{{\displaystyle 7}}-0,77 \\ =\left(-\frac{{\displaystyle 9}}{{\displaystyle 7}}\right)+\frac{{\displaystyle 2}}{{\displaystyle 7}}-1,23-0,77 \\ =-\left(\frac{{\displaystyle 9}}{{\displaystyle 7}}-\frac{{\displaystyle 2}}{{\displaystyle 7}}\right)-(1,23+0,77) \\ = (- 1) – 2 = – (1 + 2) = – 3. \end{gathered}$$