Determine the principal value of cos-1(-1/2).

Let us assume that, y = cos^-1( -1/2)

We can write this as:

cos y =- 1/2

cos y = cos (2π/3).

Thus, the Range of the principal value of cos^-1 is [0, π ].

Therefore, the principal value of cos^-1( -1/2) is 2π /3.