Question

Use the Ratio Test to determine the convergence or divergence of the series. n=1n34n\sum_{n=1}^{\infty} \frac{n^{3}}{4^{n}}

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limnan+1an=limn(n+1)34n+1÷n34n=limn(n+1)34n3=14\begin{align*} \lim\limits_{n \to \infty} \left| \dfrac{a_{n+1}}{a_{n}}\right| &= \lim\limits_{n \to \infty} \left| \dfrac{(n+1)^3}{4^{n+1}}\div \dfrac{n^3}{4^n} \right| \\ &= \lim\limits_{n \to \infty} \dfrac{(n+1)^3}{4n^3} \\ &= \dfrac{1}{4} \end{align*}

an=n34na_{n} = \dfrac{n^3}{4^n}, now we look for the limit of an+1a_{n+1}/ana_{n}.

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