Maths Olympiad Prep

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Number theory Difficulty 4.0 AMC 10/12 Find the answer Italy

Problem:

Determine what is the greatest common divisor among all the numbers that can be written as the sum of 2002 consecutive odd numbers, all positive and less than 10000 (two odd numbers are said to be consecutive if they differ by 2).

Pick one

Solution

Solution:

The answer is (D). Let us call S(n)S(n) the sum of 2002 consecutive odd numbers starting from nn.
S(1)=1+3++4003=(1+4003)+(3+4001)++(2001+2003)=40041001. S(1)=1+3+\ldots+4003=(1+4003)+(3+4001)+\ldots+(2001+2003)=4004 \cdot 1001 .
We note that S(n+2)S(n+2) and S(n)S(n) have 2001 addends in common and that their difference is therefore equal to
((n+2)+4002)n=4004 ((n+2)+4002)-n=4004
The greatest common divisor among the S(n)S(n) is therefore 4004.

SECOND SOLUTION.
The sum of the first nn odd numbers from 1 to 2n12 n-1 is n2n^{2}, so the sum of 2002 consecutive odd numbers starting from 2n+12 n+1 is (n+2002)2n2=2002(2n+2002)=4004(n+1001)(n+2002)^{2}-n^{2}=2002(2 n+2002)=4004(n+1001) so they are all multiples of 4004, and two consecutive ones have GCD 4004 because GCD(k,k+1)=1\operatorname{GCD}(k, k+1)=1.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.