2022 AMC 12B Problem 3

Attempt Problem 3 of the 2022 AMC 12B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2022 AMC 12B solutions, or check the answer key.

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3.

How many of the first ten numbers of the sequence 121,11211,1112111,121, 11211, 1112111, \ldots are prime numbers?

00

11

22

33

44

Answer: A
Concepts:factoringprime
Difficulty rating: 1350
Solution:

The nnth term consists of nn ones, then a 2,2, then nn ones. It factors as a repunit times a number of the form 10n+1:10^n + 1: 121=1111,11211=111101,1112111=11111001, \begin{aligned} &121 = 11 \cdot 11, \\ &\quad 11211 = 111 \cdot 101, \\ &\quad 1112111 = 1111 \cdot 1001, \end{aligned} and in general the nnth term equals 11n+1(10n+1).\underbrace{1\cdots1}_{n+1} \cdot (10^n + 1).

For every n1n \ge 1 both factors exceed 1,1, so every term is composite. None of the ten numbers is prime.

Thus, the correct answer is A.

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