9
a,b
This was another implementation exercise, i.e., can you implement this algorithm as specified? So pretty easy. a) took me about 30 minutes or so to code up, b) took like a minute after that since the problem is so similar.
Interestingly, the problem itself is based on a method for finding closed-form general formulae for progressions of integers. You can use it to develop a formula for sums of squares, cubes, quartics etc. or any progression that doesn't seem to have an obvious closed form.
Anyway, there is probably a language where this problem can be solved with a one-liner, I got kinda close, see 9.dart. Check the commits if it's unreadable.