Question

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

Options

• `pi/3`

• `pi/6`

• `(2pi)/3`

• `(3pi)/2`

Answer 1

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`