[artty60]

ข้อ1.

$S_n=7+77+777+7777+...$

$\frac{9}{7}S_n=\sum_{n = 1}^{n} (10^n-1)$

$S_n=\frac{7}{9}[\sum_{n = 1}^{n} (10^n-1)]$

$S_n=\frac{7}{9}(\frac{10(10^{n}-1)}{9}-n)$

$S_n=\frac{7}{81}(10^{n+1}-9n-10)$