First slide

Properties of ITF

Question

The value of $\cos^{- 1} (\cos (- 680^{\circ}))$ ,is

Moderate

A

$\frac{π}{9}$
B

$\frac{2 π}{9}$
C

$\frac{4 π}{9}$
D

$- \frac{π}{9}$

Solution

$\cos^{- 1} \{\cos (- 680^{\circ})\} = \cos^{- 1} (\cos 680^{\circ})$
$\begin{array}{l} = \cos^{- 1} \{\cos (680 \times \frac{π}{180})\} = \cos^{- 1} (\cos \frac{34 π}{9}) \\ = \cos^{- 1} \{\cos (4 π - \frac{2 π}{9})\} = \cos^{- 1} (\cos \frac{2 π}{9}) = \frac{2 π}{9} \end{array}$

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