What are the x-coordinates for the maximum points in the function f(x) = 4 cos(2x − π) from x = 0 to x = 2π?

Hello :
x-coordinates for the maximum points in the function f(x) = 4 cos(2x − π) from x = 0 to x = 2πis solutions of : f'(x) =0
f'(x) = - 2sin(2x − π)
- 2sin(2x − π) =0 
sin (2x − π) =0 
2x − π = kπ ...  k in Z
 x=(k+1)π/2(general solutions in  R)
from x = 0 to x = 2π :
k =0  : x = π/2
k=1 : x= π
k=2 : x= 3π/2
k = 3 : x=2π
Remains to be determined whether it is a maximum or minumum