On the remarkable properties of the pentagonal numbers

In this paper Euler considers the properties of the pentagonal numbers, those numbers of the form $\frac{3n^2\pm n}{2}$. He recalls that the infinite product $(1-x)(1-x^2)(1-x^3)\mathrm{.}\mathrm{.}\mathrm{.}$ expands into an infinite series with exponents the pentagonal numbers, and tries substituting the roots of this infinite product into this infinite series. I am not sure what he is doing in some parts: in particular, he does some complicated calculations about the roots of unity and sums of them, their squares, reciprocals, etc., and also sums some divergent series such as 1-1-1+1+1-1-1+1+…, and I would appreciate any suggestions or corrections about these parts.