Tính a) 2/15 + (-5/24); b) (-5/9) - (-7/27); c) (-7/12) + 0,75

* Hướng dẫn giải

a) $\frac{2}{15}+\left(\frac{-5}{24}\right)=\frac{16}{120}+\left(\frac{-25}{120}\right)=\frac{-9}{120}=\frac{-3}{40}$

b) $\left(\frac{-5}{9}\right)-\left(-\frac{7}{27}\right)=\left(\frac{-15}{27}\right)-\left(-\frac{7}{27}\right)=\frac{(-15)-(-7)}{27}=\frac{-8}{27}$

$c) \left(\frac{-7}{12}\right)+0,75=\left(\frac{-7}{12}\right)+\frac{75}{100}=\left(\frac{-7}{12}\right)+\frac{9}{12}=\frac{2}{12}=\frac{1}{6}$

$d) \left(\frac{-5}{9}\right)-1,25=\left(\frac{-5}{9}\right)-\frac{125}{100}=\left(\frac{-5}{9}\right)-\frac{5}{4}=\left(\frac{-20}{36}\right)-\frac{45}{36}=\frac{-65}{36}$

$e) 0,34\cdot \left(\frac{-5}{17}\right)=\frac{34}{100}\cdot \left(\frac{-5}{17}\right)=\frac{17}{50}\cdot \left(\frac{-5}{17}\right)=\frac{-5}{50}=\frac{-1}{10}$

$g) \frac{4}{9}:\left(\frac{-8}{15}\right)=\frac{4}{9}\cdot \left(\frac{-15}{8}\right)=\frac{4.\left(-15\right)}{9.8}=\frac{-5}{3.2}=\frac{-5}{6}$

$h) \left(1\frac{2}{3}\right):\left(2\frac{1}{2}\right)=\left(\frac{1.3+2}{3}\right):\left(\frac{2.2+1}{2}\right)=\frac{5}{3}:\frac{5}{2}=\frac{5}{3}\cdot \frac{2}{5}=\frac{2}{3}$

$i) \frac{2}{5}\cdot \left(-1,25\right)=\frac{2}{5}\cdot \frac{-125}{100}=\frac{2}{5}\cdot \frac{-5}{4}=\frac{-1}{2}$

$k) \left(\frac{-3}{5}\right)\cdot \left(\frac{15}{-7}\right)\cdot 3\frac{1}{9}=\left(\frac{-3}{5}\right)\cdot \left(\frac{15}{-7}\right)\cdot \frac{28}{9}=\frac{(-3)\mathrm{.15.28}}{5.(-7).9}=\frac{(-3)\mathrm{.3.5.7.4}}{5.(-7).9}=4$