- #1
R Letter
- 2
- 1
Homework Statement
Σ(n=0 to ∞) ((20)(-1)^n(x^(3n))/8^(n+1)
Homework Equations
Ratio test for Power Series: ρ=lim(n->∞) a_(n+1)/a_n
The Attempt at a Solution
I tried the ratio test for Power Series and it went like this:
ρ=lim(n->∞) (|x|^(3n+1)*8^(n+1))/(|x|^(3n)*8^(n+2))
=20|x|/8 lim(n->∞) 1
=20|x|/8
20|x|/8<1
|x|<2/5
So radius of convergence=2/5. However when I input the power series into Mathematica, it says that the radius of convergence should be 2 (http://goo.gl/9nAHoS)
Where am I going wrong in my calculation?