Math Problem Statement

solve this please

Solution

To find the sum of the series k=13n25n\sum_{k=1}^\infty \frac{3n - 2}{5^n}, we will approach it by splitting it into two separate sums and then finding each of those sums individually.

Let's rewrite the given series as: 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}

First, we evaluate the second sum: 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}. The sum of an infinite geometric series is given by: S=a1r=25115=2545=24=12S = \frac{a}{1 - r} = \frac{\frac{2}{5}}{1 - \frac{1}{5}} = \frac{\frac{2}{5}}{\frac{4}{5}} = \frac{2}{4} = \frac{1}{2}

Now, we need to evaluate the first sum: 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)2\sum_{n=1}^\infty \frac{n}{5^n} = \frac{r}{(1 - r)^2} where r=15r = \frac{1}{5}. Therefore: n=1n5n=15(115)2=15(45)2=151625=2520=54\sum_{n=1}^\infty \frac{n}{5^n} = \frac{\frac{1}{5}}{(1 - \frac{1}{5})^2} = \frac{\frac{1}{5}}{(\frac{4}{5})^2} = \frac{\frac{1}{5}}{\frac{16}{25}} = \frac{25}{20} = \frac{5}{4}

Therefore, we have: n=13n5n=3×54=154\sum_{n=1}^\infty \frac{3n}{5^n} = 3 \times \frac{5}{4} = \frac{15}{4}

Putting it all together: n=13n25n=15412=15424=134\sum_{n=1}^\infty \frac{3n - 2}{5^n} = \frac{15}{4} - \frac{1}{2} = \frac{15}{4} - \frac{2}{4} = \frac{13}{4}

Thus, the sum of the series is: 134\boxed{\frac{13}{4}}

Do you want more details or have any questions?

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: When dealing with series, breaking them down into simpler parts can often simplify the problem-solving process.

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Math Problem Analysis

Mathematical Concepts

Series
Geometric Series
Infinite Series

Formulas

Sum of a geometric series
Sum involving a geometric progression

Theorems

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Suitable Grade Level

Advanced High School