One more Limit

Calculus Level 2

lim n 2 n + 3 n n = ? n N \large \lim_{n \to \infty} \sqrt[n]{2^n+3^n} = ? \quad \quad n \in \mathbb N


The answer is 3.

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2 solutions

Jon Haussmann
Sep 29, 2017

lim n 2 n + 3 n n = lim n 3 ( 2 3 ) n + 1 n = 3. \lim_{n \to \infty} \sqrt[n]{2^n + 3^n} = \lim_{n \to \infty} 3 \cdot \sqrt[n]{\left( \frac{2}{3} \right)^n + 1} = 3.

Chew-Seong Cheong
Sep 29, 2017

Relevant wiki: Stolz–Cesàro theorem

L = lim n 2 n + 3 n n = lim n exp ( ln ( 2 n + 3 n ) 1 n ) where exp ( x ) = e x = exp ( lim n ln ( 2 n + 3 n ) n ) By Stolz-Ces a ˋ ro theorem = exp ( lim n ln ( 2 n + 1 + 3 n + 1 ) ln ( 2 n + 3 n ) n + 1 n ) = exp ( lim n ln 2 n + 1 + 3 n + 1 2 n + 3 n ) = exp ( lim n ln 2 n + 1 3 n + 3 n + 1 3 n 2 n 3 n + 3 n 3 n ) = exp ( ln 0 + 3 0 + 1 ) = exp ( ln 3 ) = 3 \begin{aligned} L & = \lim_{n \to \infty} \sqrt[n]{2^n+3^n} \\ & = \lim_{n \to \infty} \exp \left(\ln (2^n+3^n)^\frac 1n\right) & \small \color{#3D99F6} \text{where }\exp(x) = e^x \\ & = \exp \left(\lim_{n \to \infty} \frac {\ln (2^n+3^n)}n \right) & \small \color{#3D99F6} \text{By Stolz-Cesàro theorem} \\ & = \exp \left(\lim_{n \to \infty} \frac {\ln (2^{n+1}+3^{n+1}) - \ln (2^n+3^n)}{n+1-n} \right) \\ & = \exp \left(\lim_{n \to \infty} \ln \frac {2^{n+1}+3^{n+1}}{2^n+3^n} \right) \\ & = \exp \left(\lim_{n \to \infty} \ln \frac {\frac {2^{n+1}}{3^n} + \frac {3^{n+1}}{3^n}}{\frac {2^n}{3^n} + \frac {3^n}{3^n}} \right) \\ & = \exp \left(\ln \frac {0+3}{0+1} \right) = \exp (\ln 3) = \boxed{3} \end{aligned}

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