Math Problem Statement

Determine whether the integral \( \int_0^{\infty} \frac{x^2}{\sqrt{2 + x^3}} \, dx \) is convergent or divergent. If it is convergent, evaluate it. (If the quantity diverges, enter DIVERGES.)

Solution

The given integral is:

0x22+x3dx\int_0^{\infty} \frac{x^2}{\sqrt{2 + x^3}} \, dx

We are asked to determine whether this integral is convergent or divergent. If it converges, we are to evaluate it.

Step 1: Check for convergence

We need to check the behavior of the integrand at both bounds: x=0x = 0 and x=x = \infty.

Behavior at x=0x = 0:

For small values of xx, let's approximate the function x22+x3\frac{x^2}{\sqrt{2 + x^3}}. As x0x \to 0:

  • x3x^3 is very small compared to 2, so 2+x32\sqrt{2 + x^3} \approx \sqrt{2}.
  • Thus, near x=0x = 0, the integrand behaves like x22\frac{x^2}{\sqrt{2}}.

This means the integrand behaves like x2x^2 as x0x \to 0. The integral of x2x^2 near 0 converges because 0ϵx2dx=x330ϵ=ϵ33\int_0^\epsilon x^2 \, dx = \frac{x^3}{3} \Big|_0^\epsilon = \frac{\epsilon^3}{3}, which tends to 0 as ϵ0\epsilon \to 0.

So, the integrand does not cause divergence at x=0x = 0.

Behavior as xx \to \infty:

Now, for large values of xx, we analyze the asymptotic behavior of the integrand. As xx \to \infty:

  • 22 becomes negligible compared to x3x^3, so 2+x3x3=x3/2\sqrt{2 + x^3} \approx \sqrt{x^3} = x^{3/2}.
  • Thus, for large xx, the integrand behaves like x2x3/2=x1/2\frac{x^2}{x^{3/2}} = x^{1/2}.

The integral of x1/2x^{1/2} as xx \to \infty is divergent, since 1x1/2dx=23x3/21=\int_1^\infty x^{1/2} \, dx = \frac{2}{3} x^{3/2} \Big|_1^\infty = \infty.

Step 2: Conclusion

Since the integrand behaves like x1/2x^{1/2} as xx \to \infty, the integral diverges at the upper bound.

Thus, the given integral diverges.

Let me know if you would like more details on the analysis, or have any further questions!

Here are some related questions to explore:

  1. What is the general criterion for testing the convergence of improper integrals?
  2. How can we use comparison tests to determine the convergence of integrals?
  3. What are the differences between improper integrals of the first and second kind?
  4. Can the integral be evaluated numerically even if it diverges analytically?
  5. What are some typical integrands that lead to divergent integrals?

Tip: When analyzing improper integrals, focus on the behavior of the integrand at the boundaries (limits) of integration, especially at infinity or near singularities.

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Math Problem Analysis

Mathematical Concepts

Improper Integrals
Asymptotic Behavior
Convergence Tests

Formulas

Integral of \( x^2 \) and \( x^{1/2} \)
Asymptotic approximation for large x

Theorems

Test for divergence using behavior at infinity

Suitable Grade Level

Undergraduate level (Calculus II or III)