Recently I remembered a property of cosine that I learned in high school, namely that the cosine function repeats every 2[itex]\pi[/itex]k times for every integer k. I was told that naturally, then, if cos(x+y)=cos(x), then y is of the form 2[itex]\pi[/itex]k, where k is an integer. However, I was looking for a formal proof of this. Can someone help me out? Thanks.(adsbygoogle = window.adsbygoogle || []).push({});

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# Proof that if cos(x+y)=cos(x), then y=2*pi*k?

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