Maths Olympiad Prep

Library / HMMT February /212 of 1302

HMMT February

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. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Notice that 2002+22002+2 divides 20022−222002^{2}-2^{2}, so any common divisor of 2002+22002+2 and 20022+22002^{2}+2 must divide (20022+2)−(20022−22)=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+2≡01^{n}+2 \equiv 0 modulo 3 . Thus, 6 divides every number in the sequence.

Want a route through all this instead of an archive? The track puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.