Answer: From the ratio test, r = 0.4. The series converges.
The given term is: [tex]a_{n}=\frac{2^{n}}{5^{\left(n+1\right)}}\cdot n[/tex]
So the next term is = [tex]a_{n+1}=\frac{2^{\left(n+1\right)}}{5^{\left(n+2\right)}}\cdot\left(n+1\right)[/tex]
The ratio test is :
[tex]\left|\frac{a_{n+1}}{a_{n}}\right|=\left|\frac{\frac{2^{\left(n+1\right)}}{5^{\left(n+2\right)}}\cdot\left(n+1\right)}{\frac{2^{n}}{5^{\left(n+1\right)}}\cdot n}\right|\\\\lim_{n\to\infty}\left|\frac{a_{n+1}}{a_{n}}\right|=lim_{n\to\infty}\left|\frac{2^{\left(n+1\right)}}{2^{n}}\cdot\frac{5^{\left(n+1\right)}}{5^{\left(n+2\right)}}\cdot\frac{\left(n+1\right)}{n}\right|[/tex]
[tex]lim_{n\to\infty}\left|\frac{a_{n+1}}{a_{n}}\right|=lim_{n\to\infty}\left|\frac{2^{\left(n+1\right)}}{2^{n}}\cdot\frac{5^{\left(n+1\right)}}{5^{\left(n+2\right)}}\cdot\frac{\left(n+1\right)}{n}\right|[/tex]
[tex]lim_{n\to\infty}\left|\frac{a_{n+1}}{a_{n}}\right|=lim_{n\to\infty}\left|2\cdot\frac{1}{5}\cdot\frac{\left(n+1\right)}{n}\right|[/tex]
[tex]lim_{n\to\infty}\left|\frac{a_{n+1}}{a_{n}}\right|=\frac{2}{5}lim_{n\to\infty}\left|1+\frac{1}{n}\right|[/tex]
[tex]lim_{n\to\infty}\left|\frac{a_{n+1}}{a_{n}}\right|=\frac{2}{5}\left(1+0\right)[/tex]
[tex]lim_{n\to\infty}\left|\frac{a_{n+1}}{a_{n}}\right|=0.4[/tex]
Since 0.4 < 1 so the series converges.
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