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.
<span>So, x = Pi * k, with k = 0, 2, 4, 6, ...
An equivalent formula is x = 2Pi * n, with n = 0, 1, 2, 3, ..</span>
<span>valid. She used a representative sample </span>because she correctly found the average and 18,000 is a reasonable number to get for a the problem
