Cosine of 210 Degrees
Jump to navigation
Jump to search
Theorem
- $\cos 210 \degrees = \cos \dfrac {7 \pi} 6 = -\dfrac {\sqrt 3} 2$
where $\cos$ denotes cosine.
Proof
\(\ds \cos 210 \degrees\) | \(=\) | \(\ds \map \cos {360 \degrees - 150 \degrees}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \cos 150 \degrees\) | Cosine of Conjugate Angle | |||||||||||
\(\ds \) | \(=\) | \(\ds -\frac {\sqrt 3} 2\) | Cosine of $150 \degrees$ |
$\blacksquare$
Sources
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 5$: Trigonometric Functions: Exact Values for Trigonometric Functions of Various Angles