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The principal solution of `cos^-1(-1/2)` is :

#### Options

`pi/3`

`pi/6`

`(2pi)/3`

`(3pi)/2`

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#### Solution

The principal solution of `cos^-1(-1/2)= ` An angle in [0,π], whose cosine is -1/2

⇒ `cos^-1(-1/2) = pi - "cos"^-1 (1/2)` .....[because cos^{-1} (-x) = π - cos x]

`= pi - pi/3 = (2pi)/3`

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