Mister Exam

Sum of series xi²



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The solution

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  oo      
 ___      
 \  `     
  \      2
  /   x*i 
 /__,     
i = 2     
$$\sum_{i=2}^{\infty} i^{2} x$$
Sum(x*i^2, (i, 2, oo))
The radius of convergence of the power series
Given number:
$$i^{2} x$$
It is a series of species
$$a_{i} \left(c x - x_{0}\right)^{d i}$$
- power series.
The radius of convergence of a power series can be calculated by the formula:
$$R^{d} = \frac{x_{0} + \lim_{i \to \infty} \left|{\frac{a_{i}}{a_{i + 1}}}\right|}{c}$$
In this case
$$a_{i} = i^{2} x$$
and
$$x_{0} = 0$$
,
$$d = 0$$
,
$$c = 1$$
then
$$1 = \lim_{i \to \infty}\left(\frac{i^{2}}{\left(i + 1\right)^{2}}\right)$$
Let's take the limit
we find
True

False
The answer [src]
oo*x
$$\infty x$$
oo*x

    Examples of finding the sum of a series