Find the remainder when $r^{13} + 1$ is divided by $r - 1$.
Solution #1

Let $f(r)=r^{13}+1$. Then, by the Remainder Theorem, the remainder when $f(r)$ is divided by $r-1$ is $f(1) = 1^{13}+1 = \boxed{2}$.

Solution #2

If you think about geometric series often, you may notice that
$$\frac{r^{13}-1}{r-1} = r^{12}+r^{11}+r^{10}+\cdots+r^2+r+1.$$Therefore, $r^{13}+1 = (r^{13}-1)+2 = (r^{12}+r^{11}+\cdots+r+1)(r-1)+2$, and so the remainder is $\boxed{2}$.