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Difference between revisions of "2022 AMC 12B Problems/Problem 16"

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Suppose <math>x</math> and <math>y</math> are positive real numbers such that
 
Suppose <math>x</math> and <math>y</math> are positive real numbers such that
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<cmath>x^y=2^{64}\text{ and }(\log_2{x})^{\log_2{y}}=2^{7}.</cmath>
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<math>x^y=2^{64}</math> and <math>(\log_2{x})^{\log_2{y}}=2^{7}</math>.
 
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What is the greatest possible value of <math>\log_2{y}</math>?
 
What is the greatest possible value of <math>\log_2{y}</math>?
  
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Let <math>x = 2^a, y = 2^b.</math> We have <math>(2^a)^{2^b} = 2^{64} \Rightarrow 2^{a\cdot 2^b} = 2^{64} \Rightarrow a\cdot 2^b = 64,</math> and <math>a^b = 128</math>.
 
Let <math>x = 2^a, y = 2^b.</math> We have <math>(2^a)^{2^b} = 2^{64} \Rightarrow 2^{a\cdot 2^b} = 2^{64} \Rightarrow a\cdot 2^b = 64,</math> and <math>a^b = 128</math>.
  
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Then, from eq 1, <math>a = 64\cdot 2^{-b},</math> and substituting in to eq 2, <math>(64\cdot 2^{-b})^b = 64^b\cdot 2^{-b^2} = 2^6b\cdot 2^{-b^2} = 2^{6b-b^2} = 2^{7}.</math> Thus, <math>6b-b^2 = 7.</math>
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Then, from eq 1, <math>a = 64\cdot 2^{-b},</math> and substituting in to eq 2, <math>(64\cdot 2^{-b})^b = 64^b\cdot 2^{-b^2} = 2^{6b}\cdot 2^{-b^2} = 2^{6b-b^2} = 2^{7}.</math> Thus, <math>6b-b^2 = 7.</math>
  
 
Solving for <math>b</math> using the quadratic formula gets <math>b = 3 \pm \sqrt{2}.</math> Since we are looking for <math>\log_2{y}</math> which equals <math>b,</math> we put <math>\boxed{\textbf{(C)} \ 3+\sqrt{2}}</math> as our answer.
 
Solving for <math>b</math> using the quadratic formula gets <math>b = 3 \pm \sqrt{2}.</math> Since we are looking for <math>\log_2{y}</math> which equals <math>b,</math> we put <math>\boxed{\textbf{(C)} \ 3+\sqrt{2}}</math> as our answer.
  
 
~sirswagger21
 
~sirswagger21
 +
 +
==Video Solution by mop 2024==
 +
https://youtu.be/ezGvZgBLe8k&t=722s
 +
 +
~r00tsOfUnity
 +
 +
==Video Solution (Just 2 min!)==
 +
https://youtu.be/WU15Q_b9EDI
 +
 +
<i>Education, the Study of Everything
 +
 +
==Video Solution(1-16)==
 +
https://youtu.be/SCwQ9jUfr0g
 +
 +
~~Hayabusa1
  
 
== See Also ==
 
== See Also ==
 
{{AMC12 box|year=2022|ab=B|num-b=15|num-a=17}}
 
{{AMC12 box|year=2022|ab=B|num-b=15|num-a=17}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Latest revision as of 22:25, 27 October 2023

Problem

Suppose $x$ and $y$ are positive real numbers such that \[x^y=2^{64}\text{ and }(\log_2{x})^{\log_2{y}}=2^{7}.\] What is the greatest possible value of $\log_2{y}$?

$\textbf{(A) }3 \qquad \textbf{(B) }4 \qquad \textbf{(C) }3+\sqrt{2} \qquad \textbf{(D) }4+\sqrt{3} \qquad \textbf{(E) }7$

Solution

Take the base-two logarithm of both equations to get \[y\log_2 x = 64\quad\text{and}\quad (\log_2 y)(\log_2\log_2 x) = 7.\] Now taking the base-two logarithm of the first equation again yields \[\log_2 y + \log_2\log_2 x = 6.\] It follows that the real numbers $r:=\log_2 y$ and $s:=\log_2\log_2 x$ satisfy $r+s=6$ and $rs = 7$. Solving this system yields \[\{\log_2 y,\log_2\log_2 x\}\in\{3-\sqrt 2, 3 + \sqrt 2\}.\] Thus the largest possible value of $\log_2 y$ is $3+\sqrt 2 \implies \boxed{\textbf {(C)}}$.

cr. djmathman

Solution 2

$x^y=2^{64} \Rightarrow y\log_2{x}=64 \Rightarrow \log_2{x}=\dfrac{64}{y}$.

Substitution into $(\log_2{x})^{\log_2{y}}=2^{7}$ yields

$(\dfrac{64}{y})^{\log_2{y}}=2^{7} \Rightarrow \log_2{y}\log_2{\dfrac{64}{y}}=7 \Rightarrow \log_2{y}(6-\log_2{y})=7 \Rightarrow \log^2_2{y}-6\log_2{y}+7=0$.

Solving for $\log_2{y}$ yields $\log_2{y}=3-\sqrt{2}$ or $3+\sqrt{2}$, and we take the greater value $\boxed{\boldsymbol{(\textbf{C})3+\sqrt{2}}}$.

~4SunnyH

Solution 3

Let $x = 2^a, y = 2^b.$ We have $(2^a)^{2^b} = 2^{64} \Rightarrow 2^{a\cdot 2^b} = 2^{64} \Rightarrow a\cdot 2^b = 64,$ and $a^b = 128$.

Then, from eq 1, $a = 64\cdot 2^{-b},$ and substituting in to eq 2, $(64\cdot 2^{-b})^b = 64^b\cdot 2^{-b^2} = 2^{6b}\cdot 2^{-b^2} = 2^{6b-b^2} = 2^{7}.$ Thus, $6b-b^2 = 7.$

Solving for $b$ using the quadratic formula gets $b = 3 \pm \sqrt{2}.$ Since we are looking for $\log_2{y}$ which equals $b,$ we put $\boxed{\textbf{(C)} \ 3+\sqrt{2}}$ as our answer.

~sirswagger21

Video Solution by mop 2024

https://youtu.be/ezGvZgBLe8k&t=722s

~r00tsOfUnity

Video Solution (Just 2 min!)

https://youtu.be/WU15Q_b9EDI

Education, the Study of Everything

Video Solution(1-16)

https://youtu.be/SCwQ9jUfr0g

~~Hayabusa1

See Also

2022 AMC 12B (Problems • Answer Key • Resources)
Preceded by
Problem 15 		Followed by
Problem 17
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 12 Problems and Solutions

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