Math Problem Statement

Evaluate integral by gamma function

Evaluate integral by gamma function

Solution

The integral in the image is:

0xdx1+x6.\int_{0}^{\infty} \frac{x \, dx}{1 + x^6}.

We can solve this integral using the Gamma function and symmetry properties.


Step-by-Step Solution:

  1. Substitute: Let u=x6u = x^6, then x=u1/6x = u^{1/6} and dx=16u5/6dudx = \frac{1}{6} u^{-5/6} \, du. Substituting into the integral:

    0xdx1+x6=0u1/616u5/61+udu.\int_{0}^{\infty} \frac{x \, dx}{1 + x^6} = \int_{0}^{\infty} \frac{u^{1/6} \cdot \frac{1}{6} u^{-5/6}}{1 + u} \, du.

    Simplify the powers of uu:

    0xdx1+x6=160u2/31+udu.\int_{0}^{\infty} \frac{x \, dx}{1 + x^6} = \frac{1}{6} \int_{0}^{\infty} \frac{u^{-2/3}}{1 + u} \, du.


  1. Use the Beta Function Relation:

    The integral 0ua(1+u)bdu\int_{0}^{\infty} \frac{u^a}{(1+u)^b} \, du can be expressed in terms of the Beta function:

    0ua1(1+u)bdu=B(a,ba),\int_{0}^{\infty} \frac{u^{a-1}}{(1+u)^b} \, du = B(a, b-a),

    where the Beta function B(x,y)B(x, y) relates to the Gamma function as:

    B(x,y)=Γ(x)Γ(y)Γ(x+y).B(x, y) = \frac{\Gamma(x) \Gamma(y)}{\Gamma(x+y)}.

    Here, a=13a = \frac{1}{3} and b=1b = 1. Thus, the integral becomes:

    160u2/31+udu=16B(13,113).\frac{1}{6} \int_{0}^{\infty} \frac{u^{-2/3}}{1+u} \, du = \frac{1}{6} B\left(\frac{1}{3}, 1 - \frac{1}{3}\right).


  1. Simplify the Beta Function:

    Substitute into the Beta function formula:

    B(13,23)=Γ(13)Γ(23)Γ(1).B\left(\frac{1}{3}, \frac{2}{3}\right) = \frac{\Gamma\left(\frac{1}{3}\right) \Gamma\left(\frac{2}{3}\right)}{\Gamma(1)}.

    Using the property Γ(1)=1\Gamma(1) = 1 and the reflection formula for the Gamma function:

    Γ(x)Γ(1x)=πsin(πx),\Gamma(x) \Gamma(1-x) = \frac{\pi}{\sin(\pi x)},

    we get:

    Γ(13)Γ(23)=πsin(π3).\Gamma\left(\frac{1}{3}\right) \Gamma\left(\frac{2}{3}\right) = \frac{\pi}{\sin\left(\frac{\pi}{3}\right)}.

    Since sin(π3)=32\sin\left(\frac{\pi}{3}\right) = \frac{\sqrt{3}}{2}, this becomes:

    Γ(13)Γ(23)=π32=2π3.\Gamma\left(\frac{1}{3}\right) \Gamma\left(\frac{2}{3}\right) = \frac{\pi}{\frac{\sqrt{3}}{2}} = \frac{2\pi}{\sqrt{3}}.


  1. Final Result:

    Substituting back, the value of the integral is:

    0xdx1+x6=16B(13,23)=162π3=π33.\int_{0}^{\infty} \frac{x \, dx}{1 + x^6} = \frac{1}{6} \cdot B\left(\frac{1}{3}, \frac{2}{3}\right) = \frac{1}{6} \cdot \frac{2\pi}{\sqrt{3}} = \frac{\pi}{3\sqrt{3}}.


Final Answer:

0xdx1+x6=π33.\int_{0}^{\infty} \frac{x \, dx}{1 + x^6} = \frac{\pi}{3\sqrt{3}}.


Would you like more details or related concepts explained?
Here are 5 related questions to explore:

  1. How is the Beta function related to the Gamma function?
  2. Can we solve this integral using residue calculus?
  3. What is the reflection formula for the Gamma function and why is it useful?
  4. How do symmetry properties simplify improper integrals?
  5. How would this integral change if the power of x6x^6 were different?

Tip: Remember, the Gamma and Beta functions are powerful tools for solving complex integrals, especially when the limits involve infinity!

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Math Problem Analysis

Mathematical Concepts

Integral Calculus
Gamma Function
Beta Function

Formulas

B(x, y) = Γ(x)Γ(y) / Γ(x+y)
Γ(x)Γ(1-x) = π / sin(πx)

Theorems

Gamma Function Properties
Beta Function Properties
Reflection Formula for Gamma Function

Suitable Grade Level

Undergraduate (Calculus II or Higher)