Which formula gives the x-coordinates of the maximum values for y = cos(x)?

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, ..