Finding x such that 1+x^2+2x^3+x^4+2x^5+x^6... Converges

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I'm trying to review some calc, I went through the series and sequence sections pretty rapidly since my courses were all quarter-length.

I want to find x such that the series converges and find the sum.


1+x^2+2x^3+x^4+2x^5+x^6...

=(1+x^2)+(2x^3+x^4)+(2x^5+x^6)...(2x^{2n+1}+x^{2n})

\Sigma_{n=0}^{\infty}\left(x^{2n}+2x^{2n+1}\right)=\Sigma_{n=0}^{\infty}\left{(x^2)^n\right+\Sigma_{n=0}^{\infty}2x(x^2)^n

So I think that the series converges for

0< \mid x^2\mid < 1 \rightarrow 0<x<1

and the sum is

\frac{1}{1-x^2}+\frac{2x}{1-x^2}=\frac{1+2x}{1-x^2}

Is this correct? I don't have solution for this one... I'm not comfortable with it.
 
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It looks almost right. In your original series there is no 2x term, but later you include it. Also the convergence is for -1 < x < 1.
 
Hmm..I'd rather rewrite this as:
2\sum_{n=0}^{\infty}x^{n}-\sum_{n}^{\infty}(x^{2})^{n}=\frac{2}{1-x}-\frac{1}{1-x^{2}}=\frac{2+2x}{1-x^{2}}-\frac{1}{1-x^{2}}=\frac{1+2x}{1-x^{2}}
 
Last edited:
mathman said:
It looks almost right. In your original series there is no 2x term, but later you include it.

The term was in the original series, I htink you mean the 2x factor was on the term. That factor came from the following algebra:

2x^{2n+1}=2xx^{2n}=2x(x^2)^n

Edit: Latex isn't working right, I typed 2xx^{2n} on the second step above...

Also the convergence is for -1 < x < 1.

That makes sense...
 
If you look at your original series there is no term there that has 2x (just 2x nothing else) in it. But if you look at your sum and put n = 0, the outcome would be 1 + 2x ... so there is a slight flaw, but easely fixed by subtracting 2x from the sigma notation.
 
AdkinsJr said:
The term was in the original series, I htink you mean the 2x factor was on the term. That factor came from the following algebra:

2x^{2n+1}=2xx^{2n}=2x(x^2)^n

Edit: Latex isn't working right, I typed 2xx^{2n} on the second step above...



That makes sense...

The 2x I was referring to was simply the missing term (as ojs pointed out).
 
mathman said:
The 2x I was referring to was simply the missing term (as ojs pointed out).

Oh yes, haha. I see now. I just typed the original series wrong. It is 1+2x+x^2+2x^3...The grouping should be (1+2x)+(x^2+2x^3)...
 
= (1 + x2 + x3 + …) + (x3 + x5 + …) :wink:
 

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