2023 AMC 12A Problems/Problem 6

Problem

Points $A$ and $B$ lie on the graph of $y=\log_{2}x$ . The midpoint of $\overline{AB}$ is $(6, 2)$ . What is the positive difference between the $x$ -coordinates of $A$ and $B$ ?

$\textbf{(A)}~2\sqrt{11}\qquad\textbf{(B)}~4\sqrt{3}\qquad\textbf{(C)}~8\qquad\textbf{(D)}~4\sqrt{5}\qquad\textbf{(E)}~9$

Solution

Let $A(6+m,2+n)$ and $B(6-m,2-n)$ , since (6,2) is their midpoint. Thus, we must find 2m. We find two equations due to $A,B$ both lying on the function $y=\log_{2}x$ . The two equations are then $\log_{2}(6+m)=2+n$ and $\log_{2}(6-m)=2-n$ . Now add these two equations to obtain $log_{2}(6+m)+log_{2}(6-m)=4$ . By logarithm rules, we get $log_{2}((6+m)(6-m))=4$ . By taking 2 to the power of both sides (what's the word for this?) we obtain $(6+m)(6-m)=16$ . We then get $36-m^2=16 \rightarrow m^2=20 \rightarrow m=2\sqrt{5}$ . Since we're looking for $2m$ , we obtain $2*2\sqrt{5}=\boxed{\textbf{(D) }4\sqrt{5}}$

~amcrunner (yay, my first AMC solution)

2023 AMC 12A (Problems • Answer Key • Resources)
Preceded by Problem 5	Followed by Problem 7
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All AMC 12 Problems and Solutions

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2023 AMC 12A Problems/Problem 6

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