Zadatu sumu možeš napisati u sledećem obliku:
[dispmath]\begin{align}
S_n=7+77+777+\cdots+7\underbrace{7777\ldots7}_{n-1\text{ sedmica}}&+7\underbrace{77777\ldots7}_{n\text{ sedmica}}\\
=7+70+700+\cdots+7\underbrace{0000\ldots0}_{n-1\text{ nula}}&+7\underbrace{00000\ldots0}_{n\text{ nula}}\\
+\enspace7+\enspace70+\cdots+\enspace7\underbrace{000\ldots0}_{n-2\text{ nula}}&+\enspace7\underbrace{0000\ldots0}_{n-1\text{ nula}}\\
+\enspace\enspace7+\cdots+\enspace\enspace7\underbrace{00\ldots0}_{n-3\text{ nula}}&+\enspace\enspace7\underbrace{000\ldots0}_{n-2\text{ nula}}\\
\vdots\\
+7&+\enspace\enspace\enspace\enspace\enspace\enspace\enspace\enspace70\\
&+\enspace\enspace\enspace\enspace\enspace\enspace\enspace\enspace\enspace7
\end{align}[/dispmath] Da li ti to daje početnu ideju?