To solve this problem you must apply the proccedure shown below:
1. You want to express Cos(7π/3) as a trigonometric function of an angle in Quadrant I. Therefore, you have:
2. You can find the coterminal angle of 7π/3 by substract 2π, as following:
=7π/3-2π
=π/3
3. Then, you have:
Cos(7π/3)
Cos(π/3)
The answer is: Cos(π/3)