What is the Result of an Infinite Series in Terms of x for x < 1?

In summary, the question asks for the result in terms of x of a given series for x < 1. The numerator is unknown, but the denominator is known to tend to 1/(1-x). The solution can be found by differentiating the series, which yields a result of x/(1-x)^2. Alternatively, the solution can be found by integrating and then differentiating the sum, which also yields the same result.
  • #1
benf.stokes
71
0

Homework Statement



Hi,

How do i determine de result in terms of x of this series for x < 1:

(Sum(i=0..+infinity; i*x^i))/(Sum(i=0..+infinity;x^i)

Thanks


The Attempt at a Solution



I know that (Sum(i=0..+infinity;x^i) will tend do 1/(1-x) but i don't know what the numerator will tend to

Thanks
 
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  • #2
Hi benf.stokes! :smile:

(have a sigma: ∑ and an infinity: ∞ and try using the X2 and X2 tags just above the Reply box :wink:)
benf.stokes said:
(Sum(i=0..+infinity; i*x^i))/(Sum(i=0..+infinity;x^i)

I know that (Sum(i=0..+infinity;x^i) will tend do 1/(1-x) but i don't know what the numerator will tend to

Thanks

Hint: integrate. :smile:
 
  • #3
Thanks, I figured it out but by differentiating (sorry for the delay but I was netless for a few days):

[tex]
\sum_{n=0}^{\infty}x^n=\frac{1}{1-x}
[/tex]

[tex]
\sum_{n=0}^{\infty}nx^{n-1}=\frac{1}{(1-x)^2}
[/tex]

[tex]
\sum_{n=0}^{\infty}nx^n=\frac{x}{(1-x)^2}
[/tex]

How would it be done by integrating? The other way around?
 
Last edited:
  • #4
Sum the integral, and then differentiate that sum. :smile:
 

What is an infinite series in terms of x?

An infinite series in terms of x is a mathematical expression that consists of an infinite number of terms, where each term is a multiple of x raised to a different power. It can be written in the form of ∑anxn, where n ranges from 0 to infinity and an represents the coefficient of each term.

What is the convergence of an infinite series in terms of x?

The convergence of an infinite series in terms of x refers to whether the series will have a finite sum or not. If the value of x is within a certain range, the series will have a finite sum and is said to converge. However, if the value of x is outside this range, the series will not have a finite sum and is said to diverge.

How can we determine the convergence of an infinite series in terms of x?

There are various methods to determine the convergence of an infinite series in terms of x. The most commonly used methods are the Ratio Test, the Root Test, and the Integral Test. These tests involve checking the limit of the ratio, root, or integral of the series, respectively, to determine whether it converges or diverges.

What is the role of x in an infinite series in terms of x?

The value of x in an infinite series in terms of x is crucial as it determines the convergence or divergence of the series. It also affects the value of the sum of the series. Different values of x can result in different convergence behavior and different sums for the same series.

What are some real-world applications of infinite series in terms of x?

Infinite series in terms of x have various applications in physics, engineering, and other fields of science. For example, Taylor series, which is a type of infinite series, is used in calculus to approximate complicated functions. This is particularly useful in physics and engineering where accurate approximations are needed. Other applications include signal processing, probability, and finance.

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