2008 AMC 12B Problem 12 — Solution | LIVE by Po-Shen Loh

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2008 AMC 12B Problem 12

Attempt Problem 12 of the 2008 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 2008 AMC 12B solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

12.

For each positive integer $n,$ the mean of the first $n$ terms of a sequence is $n.$ What is the $2008$ th term of the sequence?

$2008$

$4015$

$4016$

$4{,}030{,}056$

$4{,}032{,}064$

Answer: B

Concepts:mean summation

Difficulty rating: 1500

Solution:

Since the mean of the first $n$ terms is $n,$ their sum is $n \cdot n = n^2.$

The $n$ th term is the difference of consecutive sums, $n^2 - (n-1)^2 = 2n - 1.$

For $n = 2008,$ the term is $2 \cdot 2008 - 1 = 4015.$

Thus, the correct answer is B.

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