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

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