I'm finding myself getting back into math related stuff for the first time in a while. So please be patient with me.
How does $\frac{(n-i)(n-i+1)}{2}$ expand out to:
$\frac{n^2 - (2i - 1)n - i + i^2}{2}$
If you could show me step by step, I would really appreciate it.
Also, I apologise, but I really didn't know what to tag this with.