Which of the following is the value of √\log_2 6 + \log_3 6?

2020 AMC 12B Problem 13 — Solution

2020 AMC 12B Problem 13

Attempt Problem 13 of the 2020 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 2020 AMC 12B solutions, or check the answer key.

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

13.

Which of the following is the value of $\sqrt{\log_2 6 + \log_3 6}?$

$1$

$\sqrt{\log_5 6}$

$2$

$\sqrt{\log_2 3} + \sqrt{\log_3 2}$

$\sqrt{\log_2 6} + \sqrt{\log_3 6}$

Answer: D

Concepts:logarithm algebraic manipulation

Difficulty rating: 1590

Solution:

Let $a = \log_2 3,$ so $\log_3 2 = \tfrac1a.$ Then $\begin{gathered} \log_2 6 + \log_3 6 \\ {}= (1 + \log_2 3) \\ \quad {}+ (1 + \log_3 2) \\ {}= 2 + a + \frac1a. \end{gathered}$

Meanwhile $\left(\sqrt{\log_2 3} + \sqrt{\log_3 2}\right)^2$ $= a + \frac1a + 2\sqrt{a \cdot \tfrac1a}$ $= a + \frac1a + 2,$ which equals the expression above.

Taking square roots, $\sqrt{\log_2 6 + \log_3 6}$ $= \sqrt{\log_2 3} + \sqrt{\log_3 2}.$

Thus, the correct answer is D.

← Problem 12#12

Full Exam

Problem 14#14 →

Problem 13 in Other Years

1999 AMC 12 · 2000 AMC 12 · 2001 AMC 12 · 2002 AMC 12A · 2002 AMC 12B · 2003 AMC 12A · 2003 AMC 12B · 2004 AMC 12A · 2004 AMC 12B · 2005 AMC 12A · 2005 AMC 12B · 2006 AMC 12A · 2006 AMC 12B · 2007 AMC 12A · 2007 AMC 12B · 2008 AMC 12A · 2008 AMC 12B · 2009 AMC 12A · 2009 AMC 12B · 2010 AMC 12A · 2010 AMC 12B · 2011 AMC 12A · 2011 AMC 12B · 2012 AMC 12A · 2012 AMC 12B · 2013 AMC 12A · 2013 AMC 12B · 2014 AMC 12A · 2014 AMC 12B · 2015 AMC 12A · 2015 AMC 12B · 2016 AMC 12A · 2016 AMC 12B · 2017 AMC 12A · 2017 AMC 12B · 2018 AMC 12A · 2018 AMC 12B · 2019 AMC 12A · 2019 AMC 12B · 2020 AMC 12A · 2021 AMC 12A Spring · 2021 AMC 12B Spring · 2021 AMC 12A Fall · 2021 AMC 12B Fall · 2022 AMC 12A · 2022 AMC 12B · 2023 AMC 12A · 2023 AMC 12B · 2024 AMC 12A · 2024 AMC 12B · 2025 AMC 12A · 2025 AMC 12B