Maths Olympiad Prep

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Geometry Difficulty 6.1 National olympiad Prove it Saudi Arabia

A Geostationary Earth Orbit is situated directly above the equator and has a period equal to the Earth's rotational period. It is at the precise distance of 22,23622,236 miles above the Earth that a satellite can maintain an orbit with a period of rotation around the Earth exactly equal to 2424 hours. Because the satellites revolve at the same rotational speed of the Earth, they appear stationary from the Earth surface. That is why most station antennas (satellite dishes) do not need to move once they have been properly aimed at a target satellite in the sky. In an international project, a total of ten stations were equally spaced on this orbit (at the precise distance of 22,23622,236 miles above the equator). Given that the radius of the Earth is 39603960 miles, find the exact straight distance between two neighboring stations. Write your answer in the form a+bca+b \sqrt{c}, where a,b,ca, b, c are integers and c>0c>0 is square-free.

Solution

Let AA and BB be two neighboring stations. We have AOB^=π5\widehat{A O B}=\frac{\pi}{5}, hence AB=2Rsinπ10A B=2 R \sin \frac{\pi}{10}, where R=22236+3960=26196R=22236+3960=26196. We will prove that sinπ10=514\sin \frac{\pi}{10}=\frac{\sqrt{5}-1}{4}.

Figure 1

Since sinπ=0\sin \pi=0, then sin(2π5+3π5)=0\sin \left(\frac{2 \pi}{5}+\frac{3 \pi}{5}\right)=0. We have:
sin2π5cos3π5+cos2π5sin3π5=02sinπ5cosπ5(4cos3π53cosπ5)+(2cos2π51)(3sinπ54sin3π5)=0 \begin{gathered} \sin \frac{2 \pi}{5} \cos \frac{3 \pi}{5}+\cos \frac{2 \pi}{5} \sin \frac{3 \pi}{5}=0 \Rightarrow \\ 2 \sin \frac{\pi}{5} \cos \frac{\pi}{5}\left(4 \cos ^{3} \frac{\pi}{5}-3 \cos \frac{\pi}{5}\right) \\ +\left(2 \cos ^{2} \frac{\pi}{5}-1\right)\left(3 \sin \frac{\pi}{5}-4 \sin ^{3} \frac{\pi}{5}\right)=0 \end{gathered}
We may divide by sinπ5\sin \frac{\pi}{5} and we get
2cosπ5(4cos3π53cosπ5)+(2cos2π51)(1+4cos2π5)=0 \begin{gathered} 2 \cos \frac{\pi}{5}\left(4 \cos ^{3} \frac{\pi}{5}-3 \cos \frac{\pi}{5}\right) \\ +\left(2 \cos ^{2} \frac{\pi}{5}-1\right)\left(-1+4 \cos ^2 \frac{\pi}{5}\right)=0 \end{gathered}
Denote y=cosπ5y=\cos \frac{\pi}{5}. Then, our equation becomes
16y412y2+1=0 16 y^{4}-12 y^{2}+1=0
with the solutions given by y2=3±58y^{2}=\frac{3 \pm \sqrt{5}}{8}. Since π6<π5<π4\frac{\pi}{6}<\frac{\pi}{5}<\frac{\pi}{4}, then cosπ5(22,32)\cos \frac{\pi}{5} \in\left(\frac{\sqrt{2}}{2}, \frac{\sqrt{3}}{2}\right), so that
y2=3+58 and cosπ5=3+58=5+14 y^{2}=\frac{3+\sqrt{5}}{8} \text{ and } \cos \frac{\pi}{5}=\sqrt{\frac{3+\sqrt{5}}{8}}=\frac{\sqrt{5}+1}{4}
Now, sinπ10=1cosπ52=514\sin \frac{\pi}{10}=\sqrt{\frac{1-\cos \frac{\pi}{5}}{2}}=\frac{\sqrt{5}-1}{4}. It follows
AB=26196512=13098+130985 A B=26196 \cdot \frac{\sqrt{5}-1}{2}=-13098+13098 \sqrt{5}

Figure 1

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.