- #1
DottZakapa
- 239
- 17
- Homework Statement
- radius of convergence of the following power series
##\sum_{k=0}^\infty \frac {2^n+3^n}{4^n+5^n} x^n##
- Relevant Equations
- convergence tests
##\sum_{k=0}^\infty \frac {2^n+3^n}{4^n+5^n} x^n##
in order to find the radius of convergence i apply the root test, that is
##\lim_{n \rightarrow +\infty} \sqrt [n]\frac {2^n+3^n}{4^n+5^n}##
##\lim_{n \rightarrow +\infty} \left(\frac {2^n+3^n}{4^n+5^n}\right)^\left(\frac 1 n\right)=\lim_{n \rightarrow +\infty} \left(\frac {3^n \left(\frac {2^n}{3^n}+1\right)}{5^n\left(\frac{4^n }{5^n}+1\right)}\right)^\left(\frac 1 n\right)= \lim_{n \rightarrow +\infty} \left(\frac {3} {5}\right) \left(\frac {\left(\left(\frac {2} {3}\right)^n +1\right)} {\left(\left(\frac{4 } {5}\right)^n+1 \right)}\right)^\left (\frac 1 n \right)=\left(\frac {3} {5}\right) \lim_{n \rightarrow +\infty} \left(\frac {\left(\left(\frac {2} {3}\right)^n +1\right)^\left (\frac 1 n \right)} {\left(\left(\frac{4 } {5}\right)^n+1 \right)^\left (\frac 1 n \right)}\right)##
at this point i am stuck, don't know how to handle it, probably i am forgetting some properties in order to simplify, supposing that up to here i did it correctly.
in order to find the radius of convergence i apply the root test, that is
##\lim_{n \rightarrow +\infty} \sqrt [n]\frac {2^n+3^n}{4^n+5^n}##
##\lim_{n \rightarrow +\infty} \left(\frac {2^n+3^n}{4^n+5^n}\right)^\left(\frac 1 n\right)=\lim_{n \rightarrow +\infty} \left(\frac {3^n \left(\frac {2^n}{3^n}+1\right)}{5^n\left(\frac{4^n }{5^n}+1\right)}\right)^\left(\frac 1 n\right)= \lim_{n \rightarrow +\infty} \left(\frac {3} {5}\right) \left(\frac {\left(\left(\frac {2} {3}\right)^n +1\right)} {\left(\left(\frac{4 } {5}\right)^n+1 \right)}\right)^\left (\frac 1 n \right)=\left(\frac {3} {5}\right) \lim_{n \rightarrow +\infty} \left(\frac {\left(\left(\frac {2} {3}\right)^n +1\right)^\left (\frac 1 n \right)} {\left(\left(\frac{4 } {5}\right)^n+1 \right)^\left (\frac 1 n \right)}\right)##
at this point i am stuck, don't know how to handle it, probably i am forgetting some properties in order to simplify, supposing that up to here i did it correctly.