What is the exact value of cos 2π/3.

The problem ask to calculate the exact value of the expression or trigonometric expression that is stated in you problem. Base on following some theories and rules about solving trigonometric expressions, the value of cos 2pi over 3 would be - 1/2. I hope you are satisfied with my answer and feel free to ask for more

Cos 2 (phi) / 3 = cos 120° = - cos 60° = - 1/2