Cosine of Three Right Angles

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Theorem

$\cos 270 \degrees = \cos \dfrac {3 \pi} 2 = 0$

where $\cos$ denotes cosine.


Proof

\(\ds \cos 270 \degrees\) \(=\) \(\ds \map \cos {360 \degrees - 90 \degrees}\)
\(\ds \) \(=\) \(\ds \cos 90 \degrees\) Cosine of Conjugate Angle
\(\ds \) \(=\) \(\ds 0\) Cosine of Right Angle

$\blacksquare$


Also see


Sources

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