2016 AMC 12A Problem 16

Attempt Problem 16 of the 2016 AMC 12A 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 2016 AMC 12A solutions, or check the answer key.

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16.

The graphs of y=log3x,y=\log_3 x, y=logx3,y=\log_x 3, y=log1/3x,y=\log_{1/3} x, and y=logx13y=\log_x\dfrac{1}{3} are plotted on the same set of axes. How many points in the plane with positive xx-coordinates lie on two or more of the graphs?

22

33

44

55

66

Answer: D
Concepts:logarithmsubstitution
Difficulty rating: 1860
Solution:

Let u=log3x.u=\log_3 x. Then logx3=1u,\log_x 3=\dfrac1u, log1/3x=u,\log_{1/3}x=-u, and logx13=1u.\log_x\dfrac13=-\dfrac1u. Two graphs meet where two of u,1u,u,1uu,\dfrac1u,-u,-\dfrac1u are equal for some valid x>0.x\gt 0.

Setting u=1uu=\dfrac1u gives u=±1,u=\pm1, so x=3x=3 or x=13;x=\dfrac13; setting u=1u-u=-\dfrac1u gives the same values. Setting u=uu=-u gives u=0,u=0, i.e. x=1,x=1, where log3x\log_3 x and log1/3x\log_{1/3}x are both 0.0. The remaining pairings have no real solution.

The distinct intersection points are (1,0),(1,0), (3,1),(3,1), (13,1),\left(\dfrac13,-1\right), (3,1),(3,-1), and (13,1),\left(\dfrac13,1\right), so there are 5.5.

Thus, the correct answer is D.

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