2021 AMC 12A Fall Problem 9

Attempt Problem 9 of the 2021 AMC 12A Fall 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 2021 AMC 12A Fall solutions, or check the answer key.

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9.

A right rectangular prism whose surface area and volume are numerically equal has edge lengths log2x,log3x,\log_2 x, \log_3 x, and log4x.\log_4 x. What is x?x?

262\sqrt{6}

666\sqrt{6}

2424

4848

576576

Answer: E
Concepts:surface areavolumelogarithm
Difficulty rating: 1500
Solution:

Let a=log2x,a = \log_2 x, b=log3x,b = \log_3 x, c=log4x.c = \log_4 x. Surface area equals volume gives 2(ab+bc+ca)=abc.2(ab + bc + ca) = abc. Dividing by abc,abc, 1=2(1c+1a+1b). 1 = 2\left(\frac{1}{c} + \frac{1}{a} + \frac{1}{b}\right).

Since 1a=logx2,\dfrac{1}{a} = \log_x 2, etc., the sum is logx2+logx3+logx4=logx24.\log_x 2 + \log_x 3 + \log_x 4 = \log_x 24. Thus 1=2logx24,1 = 2\log_x 24, so logx24=12,\log_x 24 = \tfrac{1}{2}, meaning x1/2=24x^{1/2} = 24 and x=576.x = 576.

Thus, the correct answer is E.

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