Math Problem Statement

What is the limit of (n√(n+5) - 3√n) / (√(n³ - 2) + √n³) as n tends to infinity?

Solution

The mathematical expression in the image involves a limit. Here is what I can interpret from it:

limnnn+53nn32+n3.\lim_{n \to \infty} \frac{n\sqrt{n+5} - 3\sqrt{n}}{\sqrt{n^3 - 2} + \sqrt{n^3}}.

To solve this limit as nn \to \infty, let's break it down step by step:

Step 1: Simplify the expression

  1. Numerator: nn+53nn\sqrt{n+5} - 3\sqrt{n}

    We can rewrite nn+5n\sqrt{n+5} as nn(1+5n)=nn1+5nn\sqrt{n(1 + \frac{5}{n})} = n\sqrt{n}\sqrt{1 + \frac{5}{n}}.

    For large nn, 1+5n1+52n\sqrt{1 + \frac{5}{n}} \approx 1 + \frac{5}{2n} (using a first-order binomial expansion).

    Therefore: nn+5nn(1+52n)=nn+5n2.n\sqrt{n+5} \approx n\sqrt{n} \left(1 + \frac{5}{2n}\right) = n\sqrt{n} + \frac{5\sqrt{n}}{2}.

    So, the numerator becomes: nn+53nnn+5n23n=nnn2.n\sqrt{n+5} - 3\sqrt{n} \approx n\sqrt{n} + \frac{5\sqrt{n}}{2} - 3\sqrt{n} = n\sqrt{n} - \frac{\sqrt{n}}{2}.

Step 2: Simplify the denominator

  1. Denominator: n32+n3\sqrt{n^3 - 2} + \sqrt{n^3}

    For large nn, n32n3=n3/2\sqrt{n^3 - 2} \approx \sqrt{n^3} = n^{3/2}, because the term 2-2 becomes negligible as nn \to \infty.

    Therefore: n32+n32n3/2.\sqrt{n^3 - 2} + \sqrt{n^3} \approx 2n^{3/2}.

Step 3: Compute the limit

Now, substitute the simplified expressions back into the limit:

limnnnn22n3/2.\lim_{n \to \infty} \frac{n\sqrt{n} - \frac{\sqrt{n}}{2}}{2n^{3/2}}.

Factor n\sqrt{n} from the numerator:

limnn(n12)2n3/2=limnn122n.\lim_{n \to \infty} \frac{\sqrt{n} \left(n - \frac{1}{2}\right)}{2n^{3/2}} = \lim_{n \to \infty} \frac{n - \frac{1}{2}}{2n}.

As nn \to \infty, n122n12\frac{n - \frac{1}{2}}{2n} \to \frac{1}{2}.

Final Answer:

12.\boxed{\frac{1}{2}}.

Would you like further details on any specific steps or have questions about this solution?

Here are 5 related questions for practice:

  1. How would the result change if the denominator had different terms, like n3+10\sqrt{n^3 + 10}?
  2. Can you explain why the binomial expansion is useful for approximating square roots at large nn?
  3. What happens if the powers of nn in the numerator and denominator were different?
  4. How do we justify neglecting small terms like 2-2 in large nn approximations?
  5. Can L'Hopital's Rule be applied here, and if so, how?

Tip: When dealing with limits at infinity, identifying dominant terms (those with the highest power of nn) helps simplify the calculation!

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Math Problem Analysis

Mathematical Concepts

Limits
Asymptotic Analysis
Binomial Expansion

Formulas

√(1 + x) ≈ 1 + x/2 for small x
Limit of a rational function

Theorems

Binomial Approximation for large n
Simplification of dominant terms

Suitable Grade Level

Undergraduate Calculus