Tính a) 0,75 - 5/6 + 1 1/2; b) 3/7 + 4/15 + (-8/21) + (-0,4);

* Hướng dẫn giải

a) $0,75-\frac{5}{6}+1\frac{1}{2}=\frac{75}{100}-\frac{5}{6}+\frac{3}{2}=\frac{3}{4}-\frac{5}{6}+\frac{3}{2}$

$$\begin{gathered} b) \frac{3}{7}+\frac{4}{15}+\left(\frac{-8}{21}\right)+(-0,4)=\frac{3}{7}+\left(\frac{-8}{21}\right)+\frac{4}{15}+\frac{-4}{10} \\ =\left(\frac{9}{21}+\frac{-8}{21}\right)+\frac{4}{15}+\frac{-2}{5}=\frac{1}{21}+\frac{4}{15}+\frac{-6}{15}=\frac{1}{21}+\frac{-2}{15}=\frac{5}{105}+\frac{-14}{105}=\frac{-9}{105}=\frac{-3}{35} \end{gathered}$$

$$\begin{gathered} c) 0,625+\left(\frac{-2}{7}\right)+\frac{3}{8}+\left(\frac{-5}{7}\right)+1\frac{2}{3}=\frac{625}{1000}+\left(\frac{-2}{7}\right)+\frac{3}{8}+\left(\frac{-5}{7}\right)+\frac{5}{3} \\ =\frac{5}{8}+\left(\frac{-2}{7}\right)+\frac{3}{8}+\left(\frac{-5}{7}\right)+\frac{5}{3}=\left(\frac{5}{8}+\frac{3}{8}\right)+\left[\left(\frac{-2}{7}\right)+\left(\frac{-5}{7}\right)\right]+\frac{5}{3} \\ =1+(-1)+\frac{5}{3}=\frac{5}{3} \end{gathered}$$

$$\begin{gathered} d) (-3)\cdot \left(\frac{-38}{21}\right)\cdot \left(\frac{-7}{6}\right)\cdot \left(-\frac{3}{19}\right)=\frac{(-3).(-38).(-7).(-3)}{\mathrm{21.6.19}} \\ =\frac{(-3).(-7).(-38).(-3)}{\mathrm{21.6.19}}=\frac{\mathrm{3.7.2.19.3}}{\mathrm{3.7.2.3.19}}=1 \end{gathered}$$

$e) \left(\frac{11}{18}:\frac{22}{9}\right)\cdot \frac{8}{5}=\frac{11}{18}\cdot \frac{9}{22}\cdot \frac{8}{5}=\frac{\mathrm{11.9.8}}{\mathrm{9.2.11.2.5}}=\frac{2}{5}$

$g) \left[\left(\frac{-4}{5}\right)\cdot \frac{5}{8}\right]:\left(\frac{-25}{12}\right)=\frac{\left(-4\right).5}{5.8}:\left(\frac{-25}{12}\right)=\frac{-1}{2}:\left(\frac{-25}{12}\right)=\frac{-1}{2}.\frac{12}{-25}=\frac{6}{25}$