Mister Exam

# Sum of series cos(n)/n

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### The solution

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  oo
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\
\   cos(n)
)  ------
/     n
/__,
n = 1       
$$\sum_{n=1}^{\infty} \frac{\cos{\left(n \right)}}{n}$$
Sum(cos(n)/n, (n, 1, oo))
The radius of convergence of the power series
Given number:
$$\frac{\cos{\left(n \right)}}{n}$$
It is a series of species
$$a_{n} \left(c x - x_{0}\right)^{d n}$$
- power series.
The radius of convergence of a power series can be calculated by the formula:
$$R^{d} = \frac{x_{0} + \lim_{n \to \infty} \left|{\frac{a_{n}}{a_{n + 1}}}\right|}{c}$$
In this case
$$a_{n} = \frac{\cos{\left(n \right)}}{n}$$
and
$$x_{0} = 0$$
,
$$d = 0$$
,
$$c = 1$$
then
$$1 = \lim_{n \to \infty}\left(\frac{\left(n + 1\right) \left|{\frac{\cos{\left(n \right)}}{\cos{\left(n + 1 \right)}}}\right|}{n}\right)$$
Let's take the limit
we find
True

False`
The rate of convergence of the power series