S = $\frac{12}{10}$ + $\frac{12}{40}$ + $\frac{12}{88}$ + $\frac{12}{154}$ + $\frac{12}{238}$ + $\frac{12}{340}$
=$12$ . ($\frac{1}{10}$ + $\frac{1}{40}$ + .... + $\frac{1}{340}$ )
=12.($\frac{1}{2.5}$ + $\frac{1}{5.8}$ + $\frac{1}{8.11}$ + .... + $\frac{1}{17.20}$)
=12.($\frac{3}{2.5}$ + $\frac{3}{5.8}$ + $\frac{3}{8.11}$ + .... + $\frac{3}{17.20}$).$\frac{1}{3}$
=12.$\frac{1}{3}$ ( $\frac{1}{2}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{8}$ + ... + $\frac{1}{17}$ - $\frac{1}{20}$
=4.($\frac{1}{2}$ - $\frac{1}{20}$) = 4. $\frac{9}{20}$ = $\frac{9}{5}$
Vậy S = $\frac{9}{5}$
