4 Sine Pi over 10 by Cosine Pi over 5/Proof 1
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Theorem
- $4 \sin \dfrac \pi {10} \cos \dfrac \pi 5 = 1$
Proof
\(\ds \paren {z + 1} \paren {z^2 - 2 z \cos \dfrac \pi 5 + 1} \paren {z^2 - 2 z \cos \dfrac {3 \pi} 5 + 1}\) | \(=\) | \(\ds z^5 + 1\) | Complex Algebra Examples: $z^5 + 1$ | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds \paren {1 + i} \paren {i^2 - 2 i \cos \dfrac \pi 5 + 1} \paren {i^2 - 2 i \cos \dfrac {3 \pi} 5 + 1}\) | \(=\) | \(\ds i^5 + 1\) | putting $z \gets i$ | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds \paren {1 + i} \paren {-1 - 2 i \cos \dfrac \pi 5 + 1} \paren {-1 - 2 i \cos \dfrac {3 \pi} 5 + 1}\) | \(=\) | \(\ds i + 1\) | Definition of Imaginary Unit | ||||||||||
\(\ds -4 \paren {1 + i} \cos \dfrac \pi 5 \cos \dfrac {3 \pi} 5\) | \(=\) | \(\ds i + 1\) | simplifying | |||||||||||
\(\ds -4 \cos \dfrac \pi 5 \cos \dfrac {3 \pi} 5\) | \(=\) | \(\ds 1\) | equating real parts | |||||||||||
\(\ds -4 \cos \dfrac \pi 5 \cos \paren {\dfrac \pi {10} + \dfrac \pi 2}\) | \(=\) | \(\ds 1\) | ||||||||||||
\(\ds -4 \cos \dfrac \pi 5 \paren {-\sin \dfrac \pi {10} }\) | \(=\) | \(\ds 1\) | ||||||||||||
\(\ds 4 \cos \dfrac \pi 5 \sin \dfrac \pi {10}\) | \(=\) | \(\ds 1\) |
$\blacksquare$
Sources
- 1960: Walter Ledermann: Complex Numbers ... (previous) ... (next): $\S 3$. Roots of Unity: Exercise $9$