Maths Olympiad Prep

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, 2024

Geometry Difficulty 4.9 AIME Prove it United States

Problem:
A circle is tangent to both branches of the hyperbola x220y2=24x^{2}-20 y^{2}=24 as well as the xx-axis. Compute the area of this circle.

Solutions — 2

Solution 1

Solution:
Figure 1
Invert about the unit circle centered at the origin. ω\omega turns into a horizontal line, and the hyperbola turns into the following:
x2(x2+y2)220y2(x2+y2)2=24x220y2=24(x2+y2)2.24x4+(48y21)x2+24y4+20y2=0(48y21)24(24)(24y4+20y2)196y21920y2y1/2016. \begin{aligned} \frac{x^{2}}{\left(x^{2}+y^{2}\right)^{2}}-\frac{20 y^{2}}{\left(x^{2}+y^{2}\right)^{2}}=24 & \Longrightarrow x^{2}-20 y^{2}=24\left(x^{2}+y^{2}\right)^{2} . \\ & \Longrightarrow 24 x^{4}+\left(48 y^{2}-1\right) x^{2}+24 y^{4}+20 y^{2}=0 \\ & \Longrightarrow\left(48 y^{2}-1\right)^{2} \geq 4(24)\left(24 y^{4}+20 y^{2}\right) \\ & \Longrightarrow 1-96 y^{2} \geq 1920 y^{2} \\ & \Longrightarrow y \leq \sqrt{1 / 2016} . \end{aligned}
This means that the horizontal line in question is y=1/2016y=\sqrt{1 / 2016}. This means that the diameter of the circle is the reciprocal of the distance between the point and line, which is 2016\sqrt{2016}, so the radius is 504\sqrt{504}, and the answer is 504π504 \pi.

Solution 2

Solution:
Let aa be the yy-coordinate of both tangency points to the hyperbola. Then, the equation of the circle must be in the form
x220y2+c(ya)2=24. x^{2}-20 y^{2}+c(y-a)^{2}=24 .
Comparing the y2y^{2}-coefficient, we see that c=21c=21. Moreover, we need it to pass through (0,0)(0,0), so 21a2=2421 a^{2}=24. Thus, the equation of the circle is
x2+y242ay+21a2=24x2+(y21a)2=(21a)2, x^{2}+y^{2}-42 a y+21 a^{2}=24 \Longrightarrow x^{2}+(y-21 a)^{2}=(21 a)^{2},
so the radius is 21a21 a, and the area is (441a2)π=504π\left(441 a^{2}\right) \pi=504 \pi.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.