My question is about Dirichlet kernel identity. Why is the following true?
$$\sum_{k=-n}^{n}e^{ikx}=1+ 2\sum_{k=1}^{n}\cos(kx)$$
2026-05-04 11:22:58.1777893778
Dirichlet kernel identity $\sum\limits_{k=-n}^{n}e^{ikx}=1+ 2\sum_{k=1}^{n}\cos(kx)$
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Hint: $e^{ikx}+e^{i(-k)x}=2cos(kx)$, for all $1\le k\le n$.