Determine the Principal Value of cos-1(-1/2)

Answer: The value of Cos^-1 often lies between (0 and π). Here we need to see when the value of Cos x becomes -1/2. It is at Cos 2π/3. So we can write cos^-1(-1/2) as cos^-1 (cos 2π/3). So cos and cos^-1 will cancel each other and 2π/3 is the answer. 

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