Đáp án:

$\begin{array}{l}
a)\dfrac{{3.4 + 3.7}}{{6.5 + 9}} = \dfrac{{3.\left( {4 + 7} \right)}}{{3.2.5 + 3.3}}\\
 = \dfrac{{3.11}}{{3.\left( {10 + 3} \right)}}\\
 = \dfrac{{11}}{{13}}\\
\dfrac{{6.9 + 81}}{{63.3 + 9.11}} = \dfrac{{9.\left( {6 + 9} \right)}}{{9.\left( {7.3 + 11} \right)}} = \dfrac{{15}}{{32}}\\
 \Rightarrow MSC = 32.13 = 416\\
 \Rightarrow \left\{ \begin{array}{l}
\dfrac{{3.4 + 3.7}}{{6.5 + 9}} = \dfrac{{11}}{{13}} = \dfrac{{352}}{{416}}\\
\dfrac{{6.9 + 81}}{{63.3 + 9.11}} = \dfrac{{15}}{{32}} = \dfrac{{195}}{{416}}
\end{array} \right.\\
b)\dfrac{{17}}{{320}};\dfrac{{ - 9}}{{80}}\\
 \Rightarrow MSC = 320\\
 \Rightarrow \left\{ \begin{array}{l}
\dfrac{{17}}{{320}} = \dfrac{{17}}{{320}}\\
\dfrac{{ - 9}}{{80}} = \dfrac{{ - 36}}{{320}}
\end{array} \right.\\
c)MSC = 330\\
 \Rightarrow \left\{ \begin{array}{l}
\dfrac{{ - 7}}{{10}} = \dfrac{{ - 231}}{{330}}\\
\dfrac{1}{{33}} = \dfrac{{ - 10}}{{330}}
\end{array} \right.\\
d)MSC = 420\\
 \Rightarrow \left\{ \begin{array}{l}
\dfrac{{ - 5}}{{14}} = \dfrac{{ - 150}}{{420}}\\
\dfrac{7}{{20}} = \dfrac{{147}}{{420}}\\
\dfrac{9}{{42}} = \dfrac{{90}}{{420}}
\end{array} \right.
\end{array}$

Đáp án: a, 11/13                            b, 133/288

Giải thích các bước giải:

 a, 3 .4 + 3 .7 = 3 .(4 + 7) = 3 .11 = 33 = 11                                          

     6 .5 + 9          30 + 9       39        39    13                  

 b, 6 .9 + 81 =  53 + 81 =  133  

  63 .3 + 9 .11  189 + 99     288