I hope this helps, let me know if you need any further assistance!

In summary, To find the sum of the given infinite series (n=0)^(inf) SIGMA ((pi)cos(n))/(5^n), one must first convert the cosine term into exponents using the formula \cos(n)=\frac{(e^i)^n+(e^{-i})^n}{2}. This will allow the series to be expressed as a sum of geometric series, which can then be solved using the formula S=(a1)/(1-r).
  • #1
waealu
37
0

Homework Statement


(Sorry, but I haven't mastered using the sigma notation in these forums yet).

Find the sum of the following infinite series: (n=0)^(inf) SIGMA ((pi)cos(n))/(5^n).


Homework Equations


I tried using the formula S=(a1)/(1-r).

I know that a=pi, but I can't find "r." The equation doesn't have a consistent rate. Should I be using a different method?

Thanks.
 
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  • #2
Hi waealu!

The sum you have wrote down is not a geometric series, so the formula you mentioned is not applicable (yet).

The problem is the cos(n), that spoils the fun. But there is a way to change the cos(n) into exponents by using the formula

[tex]\cos(n)=\frac{(e^i)^n+(e^{-i})^n}{2}[/tex]

With this equation, you can express your series as a (sum of) geometric series.

Note, to display the sum with LaTeX, type

[ tex ] \sum_{n=0}^{+\infty} \frac{\pi \cos(n)}{5^n} [ /tex ]

without the spaces in the [ tex ] tags.
 

1. What is an infinite series?

An infinite series is a mathematical concept that represents the sum of an infinite number of terms. It is denoted as a sum of terms in the form of a1 + a2 + a3 + ... where a is a constant and n is the number of terms.

2. What is the sum of an infinite series?

The sum of an infinite series is the limit of the partial sums of the series as the number of terms approaches infinity. In other words, it is the value that the series approaches as more and more terms are added.

3. How do you determine if an infinite series converges or diverges?

An infinite series converges if the limit of the partial sums exists and is finite. It diverges if the limit does not exist or if it is infinite.

4. What is the difference between an arithmetic and geometric infinite series?

In an arithmetic series, each term is obtained by adding a constant value to the previous term. In a geometric series, each term is obtained by multiplying the previous term by a constant value. In both cases, the terms continue indefinitely.

5. How do you calculate the sum of an infinite series?

The sum of an infinite series can be calculated using various methods, such as the formula for the sum of a geometric series or the ratio test for convergence. In some cases, it may not be possible to find an exact value for the sum, but an approximation can be obtained by adding a finite number of terms.

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