Question 34
If $n$ is a multiple of $4,$ the sum
$$
\begin{aligned} s&=1+2i+3i^2+\cdots\\ &\quad+(n+1)i^n, \end{aligned}
$$
where $i=\sqrt{-1},$ equals:
$$
\begin{aligned} s&=1+2i+3i^2+\cdots\\ &\quad+(n+1)i^n, \end{aligned}
$$
where $i=\sqrt{-1},$ equals: