VTMO 2021 — Grade 9 — vtmo Question 16

Each $a_{i}$ is $1$ or $- 1$ and $S_{n} = a = 1 a = 2 + a = 2 a = 3 + \ldots + a_{n} a = 1$. Can it be zero for $a ) n = 30$; $b ) n = 28$?