Which formula gives the x -coordinates of the maximum values for y is equal to cosine of x ?

The cosine function reaches its maximum value, 1, whenever x is an even multiple of π: x = 2nπ for any integer n. Choice A, kπ for any integer k, also includes odd multiples of π, where cosine is −1, so it is not correct as written. Choice C states the even multiples of π and gives the correct formula.

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