3.1 Sums & Products

Perturbation by Young Gauss

What perturbation did the nine-year-old C.F. Gauss reportly use to quickly sum 30 integers in increments of 13 starting with 89?

Exercise 1
Choice 1 is a consequence of Gauss' calculation, and choice 4 is derived from it. Choice 2 is the perturbation we use to find a closed form for a geometric sum.