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junhua44842's Question
Math
Posted 5 months ago
please use the Sandwich Theorem to solve the question 1
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Answer from Sia
Verified answer
Posted 5 months ago
To solve the limit 

limn3n+4n+5nn
\lim_{n \rightarrow \infty} \sqrt[n]{3^{n} + 4^{n} + 5^{n}}


using the Sandwich Theorem (also known as the Squeeze Theorem), we start by analyzing the expression inside the root.

1. 
Identify the dominant term
: As n n  approaches infinity, the term 5n 5^{n}  will dominate the other terms 3n 3^{n}  and 4n 4^{n} . Therefore, we can establish the following inequalities:

5n3n+4n+5n3n+4n+5n
5^{n} \leq 3^{n} + 4^{n} + 5^{n} \leq 3^{n} + 4^{n} + 5^{n}


2. 
Taking the \( n \)-th root
: Now, we take the n n -th root of all parts of the inequality:

5nn3n+4n+5nn3n+4n+5nn
\sqrt[n]{5^{n}} \leq \sqrt[n]{3^{n} + 4^{n} + 5^{n}} \leq \sqrt[n]{3^{n} + 4^{n} + 5^{n}}


3. 
Simplifying the bounds
: The left side simplifies to:

5nn=5
\sqrt[n]{5^{n}} = 5


For the right side, we can factor out 5n 5^{n}  from the expression inside the root:

3n+4n+5nn=5n((35)n+(45)n+1)n=5(35)n+(45)n+1n
\sqrt[n]{3^{n} + 4^{n} + 5^{n}} = \sqrt[n]{5^{n} \left( \left( \frac{3}{5} \right)^{n} + \left( \frac{4}{5} \right)^{n} + 1 \right)} = 5 \sqrt[n]{\left( \frac{3}{5} \right)^{n} + \left( \frac{4}{5} \right)^{n} + 1}


4. 
Evaluating the limit
: As n n  approaches infinity, both (35)n \left( \frac{3}{5} \right)^{n}  and (45)n \left( \frac{4}{5} \right)^{n}  approach 0. Therefore, we have:

(35)n+(45)n+1n1
\sqrt[n]{\left( \frac{3}{5} \right)^{n} + \left( \frac{4}{5} \right)^{n} + 1} \rightarrow 1


Thus, we can conclude:

5limn3n+4n+5nn5
5 \leq \lim_{n \rightarrow \infty} \sqrt[n]{3^{n} + 4^{n} + 5^{n}} \leq 5


By the Sandwich Theorem, we find:

limn3n+4n+5nn=5
\lim_{n \rightarrow \infty} \sqrt[n]{3^{n} + 4^{n} + 5^{n}} = 5


So, the final answer is:

limn3n+4n+5nn=5
\lim_{n \rightarrow \infty} \sqrt[n]{3^{n} + 4^{n} + 5^{n}} = 5

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