- #1
NastyAccident
- 61
- 0
Homework Statement
[tex]\sum^{infinity}_{n=0}\frac{cos(x)^n}{3^n}[/tex]
Find the values of x for which the series converges.
(z,p)
Find the sum of the series for those values of x.
Homework Equations
Geometric Series Sum (a/(1-r))
Geometric Series a*r^n
The Attempt at a Solution
I believe I have b correct, so I'm going to lead with b solution:
[tex]\sum^{infinity}_{n=0}\frac{cos(x)^{n}}{3^{n}}[/tex]
[tex]\sum^{infinity}_{n=0}(\frac{cos(x)}{3})^{n}[/tex]
This series is a geometric series a = 1 and r = [tex]\frac{cos(x)}{3}[/tex].
Since, [tex]\left|r\right|=\left|\frac{cos(x)}{3}\right|<1[/tex], it converges and gives
[tex]\frac{1}{1-\frac{cos(x)}{3}}[/tex]
Attempt at a.
Since the series starts at n = 0, the first term, is always going to be 1. Find the values of x for which the series converges. Thus,
[tex]\left|\frac{cos(x)}{3}\right|<1[/tex]
[tex]-1<\frac{cos(x)}{3}<1[/tex]
[tex]-3<cos(x)<3[/tex]
[tex]arccos(-3)<x<arccos(3)[/tex]
But, the issue is the the domain of arccos(x) is -1<x<1... I'm not necessarily sure what I'm doing wrong for this particular problem.
I've tried -1/3 (since the range of cosine is -1 to 1) and -3, both were incorrect.
As always any help or further explanation is welcome and will be greatly appreciated!
NastyAccident