This power series is the Taylor series expansion for $\frac{\sin x}{x}$ at $x=0$. The general form of the Taylor series for a function $f(x)$ at $x=a$ is given by:
$$f(x) = \sum_{k=0}^{\infty} \frac{f^{(k)}(a)}{k!} (x-a)^k$$
In this case, $a=0$ and
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