Maths Olympiad Prep

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Number theory Difficulty 4.9 AIME Find the answer

Find the greatest common divisor of the numbers 2002+2,20022+2,20023+2,2002+2,2002^{2}+2,2002^{3}+2, \ldots.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Notice that 2002+22002+2 divides 20022222002^{2}-2^{2}, so any common divisor of 2002+22002+2 and 20022+22002^{2}+2 must divide (20022+2)(2002222)=6\left(2002^{2}+2\right)-\left(2002^{2}-2^{2}\right)=6. On the other hand, every number in the sequence is even, and the nnth number is always congruent to 1n+201^{n}+2 \equiv 0 modulo 3 . Thus, 6 divides every number in the sequence.

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