Prove that if a^n = 1, then the set c, c + ac + 2a, ..., c + (n-1)a is a complete set of residues (CSR) mod n. Hint: Show that these are all incongruent mod n and apply the pigeonhole principle.
Corollary: Prove that any n consecutive integers (that is, a, a+1, a+2, ..., a+n-1) form a CSR mod n.