a.3/21+4/9 b.23/55-6/11 c.2/5-(-4/35-3/5) nhớ giải thích nha sai cx đc

#andy

\[\begin{array}{l}
a)\dfrac{3}{{21}} + \dfrac{4}{9}\\
MC:63\\
 = \dfrac{{3.3}}{{21.3}} + \dfrac{{4.7}}{{9.7}}\\
 = \dfrac{9}{{63}} + \dfrac{{28}}{{63}}\\
 = \dfrac{{37}}{{63}}\\
b)\dfrac{{23}}{{55}} - \dfrac{6}{{11}}\\
MC:55\\
 = \dfrac{{23}}{{55}} - \dfrac{{6.5}}{{11.5}}\\
 = \dfrac{{23}}{{55}} - \dfrac{{30}}{{55}}\\
 =  - \dfrac{7}{{55}}\\
c)\dfrac{2}{5} - \left( { - \dfrac{4}{{35}} - \dfrac{3}{5}} \right)\\
 = \dfrac{2}{5} - \left( { - \dfrac{4}{{35}} - \dfrac{{21}}{{35}}} \right)\\
 = \dfrac{2}{5} - \left( { - \dfrac{{25}}{{35}}} \right)\\
 = \dfrac{2}{5} + \dfrac{5}{7}\\
 = \dfrac{{2.7}}{{35}} + \dfrac{{25}}{{35}}\\
 = \dfrac{{14}}{{35}} + \dfrac{{25}}{{35}}\\
 = \dfrac{{39}}{{35}}
\end{array}\]