Перепишем в виде:
[m]S=\frac{1}{6}+\frac{1}{6^2}+\frac{1}{6^2}+\frac{1}{6^3}+\frac{1}{6^3}+\frac{1}{6^3}+...+\frac{n}{6^{n}}+...[/m]
Получаем геометрические прогрессии
[m]\frac{1}{6}+\frac{1}{6^2}+\frac{1}{6^3}+...[/m] сумма равна [m]\frac{\frac{1}{6}}{1-\frac{1}{6}}[/m]
[m]\frac{1}{6^2}+\frac{1}{6^3}+...[/m] сумма равна [m]\frac{\frac{1}{6^2}}{1-\frac{1}{6}}[/m]
[m]\frac{1}{6^3}+...[/m] сумма равна [m]\frac{\frac{1}{6^3}}{1-\frac{1}{6}}[/m]
[m]5S=5\cdot \frac{\frac{1}{6}}{1-\frac{1}{6}}+5\cdot \frac{\frac{1}{6^2}}{1-\frac{1}{6}}+5\cdot \frac{\frac{1}{6^3}}{1-\frac{1}{6}}+...[/m]
[m]=1+\frac{1}{6}+\frac{1}{6^2}+...=\frac{1}{1-\frac{1}{6}}=\frac{6}{5}[/m]