Tuesday, 17 February 2015

trigonometry - Proof of a summation



Here is the problem I am stuck with: Is it true that for every positive integer n>1,
nk=1cos(2πkn)=0=nk=1sin(2πkn)
I'm imagining the unit circle and adding up the value of both trig functions separately but I cannot picture see how their sum add up to 0.



EDIT I updated the question from earlier and realized that it was my fault because of a major typo. At least it was better to point it out late than never. So how would I go about solving this question as of now?



Originally the question was: \sum\limits_{k=1}^n \frac {\cos(2πk)}{n} =0= \sum\limits_{k=1}^n \frac {\sin(2πk)}{n}



Answer



Old answer to old question:






Multiply both sides by n to get



\sum_{k=1}^n\cos(2\pi k)\stackrel?=0\stackrel?=\sum_{k=1}^n\sin(2\pi k)



It's easy enough to see that for positive integers k, \cos(2\pi k)=1 and \sin(2\pi k)=0, thus,




\sum_{k=1}^n\sin(2\pi k)=0



\sum_{k=1}^n\cos(2\pi k)=n\ne0


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