5

Part of 1952 Polish MO Finals

Problems(1)

last digit of 1 + 2 + 3 + ...+ n,

Source: Polish MO Finals 1952 p5

Prove that none of the digits

2

4

7

9

can be the last digit of a number

1 + 2 + 3 + \ldots + n,

n

is a natural number.

number theorylast digits