Maths Olympiad Prep

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Number theory Difficulty 4.7 AIME Prove it United States

Problem:

Do there exist 100 consecutive positive integers such that their sum is a prime number?

Solution

Solution:

No. Let n,n+1,,n+99n, n+1, \ldots, n+99 be any 100 consecutive positive integers. Then
n+(n+1)+(n+2)++(n+99)=100n+(1+2++99). n + (n+1) + (n+2) + \cdots + (n+99) = 100n + (1+2+\cdots+99).
However,
1+2++99=(1+99)+(2+98)+(3+97)++(49+51)+50=49100+50=50(249+1)=5099. 1+2+\cdots+99 = (1+99) + (2+98) + (3+97) + \cdots + (49+51) + 50 = 49 \cdot 100 + 50 = 50(2 \cdot 49 + 1) = 50 \cdot 99.
Thus
n+(n+1)++(n+99)=100n+5099=50(2n+99) n + (n+1) + \cdots + (n+99) = 100n + 50 \cdot 99 = 50(2n + 99)
and this is not prime.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.