[br]\displaystyle[br]\sum_{k=1}^{n}\(2coskx)=\dfrac{(sin(n+1/2)-sin(1/2))x}{sin(x/2)}[br]
[br]1+\sum_{k=1}^{n}\(2coskx)=\dfrac{xsin(n+1/2)-xsin(1/2)+sin(x/2)}{sin(x/2)}=f(x)[br]
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