\(\displaystyle \sum_{r=1}^{\infty} \frac{6^r}{(3^r - 2^r)(3^{r + 1} - 2^{r + 1})} \)
Note by
Kushagraa Aggarwal
5 years, 1 month ago
No vote yet
11 votes
Easy Math Editor
*italics*or_italics_**bold**or__bold__paragraph 1
paragraph 2
[example link](https://brilliant.org)> This is a quote# I indented these lines # 4 spaces, and now they show # up as a code block. print "hello world"2 \times 32^{34}a_{i-1}\frac{2}{3}\sqrt{2}\sum_{i=1}^3\sin \theta\boxed{123}Comments
Sort by:
Top NewestLet's try to generalize. For \(|x| < 1\) we have the following:
\(S(x) := \displaystyle \sum_{n = 1}^{\infty} \dfrac{x^n}{(1-x^n)(1-x^{n+1})}\) \(= \dfrac{1}{1-x}\displaystyle \sum_{n = 1}^{\infty} \dfrac{x^n(1-x)}{(1-x^n)(1-x^{n+1})}\)
\(= \dfrac{1}{1-x}\displaystyle \sum_{n = 1}^{\infty} \dfrac{x^n - x^{n+1}}{(1-x^n)(1-x^{n+1})}\) \(= \dfrac{1}{1-x}\displaystyle \sum_{n = 1}^{\infty} \dfrac{(1- x^{n+1}) - (1-x^n)}{(1-x^n)(1-x^{n+1})}\)
\(= \dfrac{1}{1-x}\displaystyle \sum_{n = 1}^{\infty}\left[\dfrac{1}{1-x^n} - \dfrac{1}{1-x^{n+1}}\right]\) \(= \dfrac{1}{1-x}\left[ \dfrac{1}{1-x^1} - \dfrac{1}{1-0}\right]\) \(= \dfrac{x}{(1-x)^2}\).
Now to tackle the sum in question:
\(\displaystyle\sum_{n = 1}^{\infty} \dfrac{6^n}{(3^n-2^n)(3^{n+1}-2^{n+1})}\) \(= \displaystyle\sum_{n = 1}^{\infty} \dfrac{3^{2n}\left(2/3\right)^n}{3^{2n+1}\left(1-\left(2/3\right)^n\right)\left(1-\left(2/3\right)^{n+1}\right)}\)
\(= \dfrac{1}{3}\displaystyle\sum_{n = 1}^{\infty} \dfrac{\left(2/3\right)^n}{\left(1-\left(2/3\right)^n\right)\left(1-\left(2/3\right)^{n+1}\right)}\) \(= \dfrac{1}{3}S\left(\dfrac{2}{3}\right)\) \(= \dfrac{1}{3} \dfrac{2/3}{\left(1-2/3\right)^2} = \boxed{2}\)
Log in to reply
The best part of this solution is that it's deceptively straight forward. If somebody were to see all of these "fancy" math symbols and such they would instinctively feel intimidated, but if you really look at what you're doing here it's really just very smart equation manipulation.
Log in to reply
Wow! Just great, Jimmy!
How did you even think of S(x)?
Log in to reply
When solving this question, I first did the manipulations to the summation. I then replaced \(2/3\) with \(x\) to see if there was some clever power series manipulations that could be done. As it turned out, it was just a telescoping sum, and replacing \(2/3\) with \(x\) was not really necessary. However, \(x\) is easier to type repeatedly than \(2/3\), and so, I showed the generalization.
Log in to reply
That makes sense, thank you.
Log in to reply
that is incredible genius imo, you should be proud xD
Log in to reply
Forgive my ignorance but what is S(x)? Is it the same as f(x)?
Log in to reply
\(S(x)\) is a function of \(x\) that I defined. I could have also named it \(f(x)\), or \(\xi(x)\), or even \(\clubsuit(x)\).
Log in to reply
How will we get
\( \sum_{n=1}^{\infty} \Bigg[ -\frac{1}{1-x^{n+1}} \Bigg] = -\frac{1}{1-0} \) ?
Please help , Thank You
Log in to reply
Look at the whole expression. It states
\( \sum_{n = 1}^{\infty} \frac {1}{1 - x^n} - \frac{1}{1 - x^{n + 1}}\). So, when \(n = 1\) then you have \( \frac {1}{1 - x} - \frac {1}{1 - x^2}\). Add \(n = 2\) you have \( \frac {1}{1 - x^2} - \frac {1}{1 - x^3}\). Notice how the first term of this cancels with the second term of the first. This is called a telescopic series. Thus, you will only have the first term ( \( \frac{1}{1 - x}\)) and the last term, \( \lim_{n \rightarrow \infty} - \frac{1}{1 - x^{n + 1}} = - \frac {1}{1 - 0} \).
Log in to reply
Thank you very much now I get it !
Good explanation too !
Thank You !
Log in to reply
If I'm not mistaken, this was a Putnam problem!
Log in to reply