Sum of Infinite Series of Product of Power and Cosine

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Theorem

Let $r \in \R$ such that $\size r < 1$.

Then:

\(\ds \sum_{k \mathop = 0}^\infty r^k \cos k x\) \(=\) \(\ds 1 + r \cos x + r^2 \cos 2 x + r^3 \cos 3 x + \cdots\)
\(\ds \) \(=\) \(\ds \dfrac {1 - r \cos x} {1 - 2 r \cos x + r^2}\)


Proof

From Euler's Formula:

$e^{i \theta} = \cos \theta + i \sin \theta$

Hence:

\(\ds \sum_{k \mathop = 0}^\infty r^k \cos k x\) \(=\) \(\ds \map \Re {\sum_{k \mathop = 0}^\infty r^k e^{i k x} }\)
\(\ds \) \(=\) \(\ds \map \Re {\sum_{k \mathop = 0}^\infty \paren {r e^{i x} }^k}\)
\(\ds \) \(=\) \(\ds \map \Re {\frac 1 {1 - r e^{i x} } }\) Sum of Infinite Geometric Sequence: valid because $\size r < 1$
\(\ds \) \(=\) \(\ds \map \Re {\frac {1 - r e^{-i x} } {\paren {1 - r e^{-i x} } \paren {1 - r e^{i x} } } }\)
\(\ds \) \(=\) \(\ds \map \Re {\frac {1 - r e^{-i x} } {1 - r \paren {e^{i x} + e^{- i x} } + r^2} }\)
\(\ds \) \(=\) \(\ds \map \Re {\frac {1 - r \paren {\cos x - i \sin x} } {1 - 2 r \cos x + r^2} }\) Euler's Formula: Corollary
\(\ds \) \(=\) \(\ds \dfrac {1 - r \cos x} {1 - 2 r \cos x + r^2}\) after simplification

$\blacksquare$


Also see


Sources

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