To determine the convergence or divergence of a series `sum a_n` using Root test, we evaluate a limit as:
`lim_(n-gtoo) root(n)(|a_n|)= L`
or
`lim_(n-gtoo) |a_n|^(1/n)= L`
Then, we follow the conditions:
a) `Llt1` then the series is absolutely convergent.
b) `Lgt1` then the series is divergent.
c) `L=1` or does not exist then the test is inconclusive. The series may be divergent, conditionally convergent, or absolutely convergent.
We may apply Root...
To determine the convergence or divergence of a series `sum a_n` using Root test, we evaluate a limit as:
`lim_(n-gtoo) root(n)(|a_n|)= L`
or
`lim_(n-gtoo) |a_n|^(1/n)= L`
Then, we follow the conditions:
a) `Llt1` then the series is absolutely convergent.
b) `Lgt1` then the series is divergent.
c) `L=1` or does not exist then the test is inconclusive. The series may be divergent, conditionally convergent, or absolutely convergent.
We may apply Root test on the given series `sum_(n=1)^oo 5^n/n^4` when we let: `a_n =5^n/n^4` .
Applying the Root test, we set-up the limit as:
`lim_(n-gtoo) |5^n/n^4|^(1/n) =lim_(n-gtoo) (5^n/n^4)^(1/n)`
Apply Law of Exponent: `(x/y)^n = x^n/y^n` and `(x^n)^m= x^(n*m)` .
`lim_(n-gtoo) (5^n/n^4)^(1/n) =lim_(n-gtoo) (5^n)^(1/n)/(n^4)^(1/n)`
` =lim_(n-gtoo)5^(n*1/n)/n^(4*1/n)`
` =lim_(n-gtoo)5^(n/n)/n^(4/n)`
` =lim_(n-gtoo)5^1/n^(4/n)`
` =lim_(n-gtoo)5/n^(4/n)`
Evaluate the limit.
`lim_(n-gtoo) 5/n^(4/n)=5 lim_(n-gtoo) 1/n^(4/n) `
` =5 *1/oo^(4/oo)`
` =5 *1/oo^(0)`
` =5 *1/1`
` = 5*1`
` =5`
The limit value `L =5` satisfies the condition: `Lgt1` since `5gt1` .
Conclusion: The series `sum_(n=1)^oo 5^n/n^4` is divergent.