2010 AIME I Problem 2

Below is the professionally curated solution for Problem 2 of the 2010 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2010 AIME I solutions, or check the answer key.

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Concepts:modular arithmeticpattern recognition

Difficulty rating: 1950

2.

Find the remainder when 999999999999 9’s9 \cdot 99 \cdot 999 \cdot \cdots \cdot \underbrace{99\ldots9}_{\text{999 9's}} is divided by 1000.1000.

Solution:

Work modulo 1000.1000. Every factor from the third one on ends in at least three 99s, so each is 1(mod1000).\equiv -1 \pmod{1000}. There are 999999 factors in all, hence 997997 of them are 1.\equiv -1.

The product is therefore 999(1)997891109(mod1000),\equiv 9 \cdot 99 \cdot (-1)^{997} \equiv -891 \equiv 109 \pmod{1000}, so the remainder is 109.109.

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