We are asked to find the sum of the infinite series $\sum_{k=1}^{\infty} (\frac{e}{\pi})^{k+1}$.

AnalysisInfinite SeriesGeometric SeriesConvergence
2025/3/6

1. Problem Description

We are asked to find the sum of the infinite series k=1(eπ)k+1\sum_{k=1}^{\infty} (\frac{e}{\pi})^{k+1}.

2. Solution Steps

The given series is a geometric series. A geometric series has the form k=0ark\sum_{k=0}^{\infty} ar^k, where aa is the first term and rr is the common ratio. The sum of an infinite geometric series converges to a1r\frac{a}{1-r} if r<1|r| < 1.
In this case, we have k=1(eπ)k+1\sum_{k=1}^{\infty} (\frac{e}{\pi})^{k+1}.
We can rewrite this as
k=1(eπ)k+1=k=1(eπ)(eπ)k=eπk=1(eπ)k\sum_{k=1}^{\infty} (\frac{e}{\pi})^{k+1} = \sum_{k=1}^{\infty} (\frac{e}{\pi}) (\frac{e}{\pi})^k = \frac{e}{\pi} \sum_{k=1}^{\infty} (\frac{e}{\pi})^k.
Now, k=1(eπ)k\sum_{k=1}^{\infty} (\frac{e}{\pi})^k is a geometric series with first term a=eπa = \frac{e}{\pi} and common ratio r=eπr = \frac{e}{\pi}.
Since e2.718e \approx 2.718 and π3.141\pi \approx 3.141, we have eπ<1\frac{e}{\pi} < 1. Therefore, the series converges.
The sum of k=1(eπ)k\sum_{k=1}^{\infty} (\frac{e}{\pi})^k is a1r=eπ1eπ=eπe\frac{a}{1-r} = \frac{\frac{e}{\pi}}{1-\frac{e}{\pi}} = \frac{e}{\pi - e}.
So, we have
k=1(eπ)k+1=eπk=1(eπ)k=eπeπ1eπ=eπeπe=e2π(πe)\sum_{k=1}^{\infty} (\frac{e}{\pi})^{k+1} = \frac{e}{\pi} \sum_{k=1}^{\infty} (\frac{e}{\pi})^k = \frac{e}{\pi} \cdot \frac{\frac{e}{\pi}}{1 - \frac{e}{\pi}} = \frac{e}{\pi} \cdot \frac{e}{\pi - e} = \frac{e^2}{\pi(\pi - e)}.
Alternatively, we can write the original series as k=1(eπ)k+1=k=2(eπ)k\sum_{k=1}^\infty (\frac{e}{\pi})^{k+1} = \sum_{k=2}^\infty (\frac{e}{\pi})^k.
The series k=0(eπ)k=11eπ=ππe\sum_{k=0}^\infty (\frac{e}{\pi})^k = \frac{1}{1 - \frac{e}{\pi}} = \frac{\pi}{\pi - e}.
Then k=2(eπ)k=k=0(eπ)k1eπ=ππe1eπ=π(πe)eπ(πe)πe=ee+e2ππe=e2ππe=e2π(πe)\sum_{k=2}^\infty (\frac{e}{\pi})^k = \sum_{k=0}^\infty (\frac{e}{\pi})^k - 1 - \frac{e}{\pi} = \frac{\pi}{\pi - e} - 1 - \frac{e}{\pi} = \frac{\pi - (\pi - e) - \frac{e}{\pi}(\pi - e)}{\pi - e} = \frac{e - e + \frac{e^2}{\pi}}{\pi - e} = \frac{\frac{e^2}{\pi}}{\pi - e} = \frac{e^2}{\pi(\pi - e)}.

3. Final Answer

e2π(πe)\frac{e^2}{\pi(\pi - e)}

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