Answer:
k pi for k = 0, plus-or-minus 2, plus-or-minus 4, ellipsis
Explanation:
The cosine is a maximum when its argument is at multiples of 2π.
k pi for k = 0, plus-or-minus 2, plus-or-minus 4, ellipsis
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Both of us could probably read this more easily if you would use appropriate math symbols.