Math Problem Statement

you're wrong

Solution

Let's go through the solution step-by-step again to identify the error and correct it.

The series given is: k=13n25n\sum_{k=1}^\infty \frac{3n - 2}{5^n}

First, let's correct the indices and properly handle each part of the series separately.

Step 1: Split the Series

n=13n25n=n=13n5nn=125n\sum_{n=1}^\infty \frac{3n - 2}{5^n} = \sum_{n=1}^\infty \frac{3n}{5^n} - \sum_{n=1}^\infty \frac{2}{5^n}

Step 2: Evaluate Each Series Separately

Sum 1: n=125n\sum_{n=1}^\infty \frac{2}{5^n}

This is a geometric series with the first term a=25a = \frac{2}{5} and common ratio r=15r = \frac{1}{5}: n=125n=25115=2545=24=12\sum_{n=1}^\infty \frac{2}{5^n} = \frac{\frac{2}{5}}{1 - \frac{1}{5}} = \frac{\frac{2}{5}}{\frac{4}{5}} = \frac{2}{4} = \frac{1}{2}

Sum 2: n=13n5n\sum_{n=1}^\infty \frac{3n}{5^n}

We use the formula for the sum of a series involving nn multiplied by a geometric progression: n=1n5n=r(1r)2wherer=15\sum_{n=1}^\infty \frac{n}{5^n} = \frac{r}{(1 - r)^2} \quad \text{where} \quad r = \frac{1}{5}

Thus: n=1n5n=15(115)2=15(45)2=151625=2580=516\sum_{n=1}^\infty \frac{n}{5^n} = \frac{\frac{1}{5}}{(1 - \frac{1}{5})^2} = \frac{\frac{1}{5}}{\left(\frac{4}{5}\right)^2} = \frac{\frac{1}{5}}{\frac{16}{25}} = \frac{25}{80} = \frac{5}{16}

Now, multiply this result by 3: n=13n5n=3×516=1516\sum_{n=1}^\infty \frac{3n}{5^n} = 3 \times \frac{5}{16} = \frac{15}{16}

Step 3: Combine the Results

n=13n25n=151612=1516816=716\sum_{n=1}^\infty \frac{3n - 2}{5^n} = \frac{15}{16} - \frac{1}{2} = \frac{15}{16} - \frac{8}{16} = \frac{7}{16}

Thus, the correct sum of the series is: 716\boxed{\frac{7}{16}}

Do you have any questions or would you like more details on any step?

Here are 5 more questions you can ask:

  1. How do you find the sum of a geometric series?
  2. Can you explain the steps to derive the sum formula for n=1n5n\sum_{n=1}^\infty \frac{n}{5^n}?
  3. What are other methods to solve series involving factorials?
  4. How do you approach solving an alternating series?
  5. Can you explain the concept of convergence in series?

Tip: Carefully checking each step of a series evaluation can help catch and correct errors in the calculation.

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Math Problem Analysis

Mathematical Concepts

Series
Geometric Series
Infinite Series
Summation

Formulas

Geometric series sum formula
Sum of series involving n and a geometric progression

Theorems

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Suitable Grade Level

Advanced High School