Bài giải :

(Tính)

S = $\frac{12}{10}+\frac{12}{40}+\frac{12}{88}+\frac{12}{154}+\frac{12}{238}+\frac{12}{340}$

    $\begin{align}

  & =4\times \left( \frac{3}{10}+\frac{3}{40}+\frac{3}{88}+\frac{3}{154}+\frac{3}{238}+\frac{3}{340} \right) \\

 & =4\times \left( \frac{3}{2\times 5}+\frac{3}{5\times 8}+\frac{3}{8\times 11}+\frac{3}{11\times 14}+\frac{3}{14\times 17}+\frac{3}{17\times 20} \right) \\

 & =4\times \left( \frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{17}+\frac{1}{17}-\frac{1}{20} \right) \\

 & =4\times \left( \frac{1}{2}-\frac{1}{20} \right) \\

 & =4\times \frac{9}{20} \\

 & =\frac{9}{5} \\

\end{align}$