The principal value of the expression cos - 1 [ cos ( - 680 ) ] is:

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Solution

Q.
The principal value of the expression $\cos^{- 1} [\cos (- 680 °)]$ is:

1 $\frac{2 π}{9}$

2 $\frac{- 2 π}{9}$

3 $\frac{34 π}{9}$

4 $\frac{π}{9}$

Ans.
(1)

Ans. $\frac{2 π}{9}$

Explanation:

$\cos^{- 1} [\cos (- 680 °)] = \cos^{- 1} [\cos (720 ° - 40 °)]$

$= \cos^{- 1} [\cos (- 40 °)]$

$= \cos^{- 1} [\cos (40 °)]$

$= 40 ° = \frac{2 π}{9}$

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