# Sine series for cos(x) (FOURIER SERIES)

by konradz
Tags: cosx, fourier, series, sine
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 P: 1 I was finally able to figure out how to find the sine series for cos(x), but only for [0,2pi]. A question i have though is what is the interval of validity? is it only [0,pi]? Ie if I actually had to sketch the graph of the sum of the series, on all of R, would I have cosine or just a periodic extension of cosine from [0,2pi]?
 Sci Advisor HW Helper P: 4,300 Hey Konrad, welcome to PF. I am afraid I don't entirely understand your question. You say that you have managed to write cos(x) as a(n infinite) sum of sines on the interval [0, 2pi]. But both cos(x) and the sines you used are periodic with period 2pi, aren't they? So if the infinite sum converges to cos(x) on an interval with a length of at least one period, then it converges to cos(x) everywhere, doesn't it?
 HW Helper Thanks PF Gold P: 7,575 If you have expanded cos(x) in a sine series using $p = 2\pi$ in the formula $$b_n = \frac 2 p \int_0^p \cos(x) \sin{\frac{n\pi x}{p}}\,dx$$ what you are representing is the $4\pi$ periodic odd extension of cos(x). [edit - corrected typo: bn not an]

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