You must remember that the periodicity of the function cos(x) is 2Pi.
Also you should know that the maximum value of cos(x) is obtained when x = 0.
So, the values of x for which cos(x) is maximum is x =0, 2Pi, 4 Pi, 6 Pi.
So, x = Pi k, with k = 0, 2, 4, 6, ...
An equivalent formula is x = 2Pi n, with n = 0, 1, 2, 3, ..
