Cosine of 60 Degrees

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Theorem

$\cos 60 \degrees = \cos \dfrac \pi 3 = \dfrac 1 2$

where $\cos$ denotes the cosine.


Proof

\(\ds \cos 60 \degrees\) \(=\) \(\ds \map \cos {90 \degrees - 30 \degrees}\)
\(\ds \) \(=\) \(\ds \sin 30 \degrees\) Cosine of Complement equals Sine
\(\ds \) \(=\) \(\ds \dfrac 1 2\) Sine of $30 \degrees$

$\blacksquare$


Sources

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