### Author Topic: 2.6 Q22  (Read 3211 times)

#### Kathy Ngo

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• Karma: 3 ##### 2.6 Q22
« on: November 22, 2018, 12:16:49 PM »
Can someone show me how they did (a)?
I have no idea how I'm supposed to use (8 ) or (9) to solve.

#### Ende Jin

• Sr. Member
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• Karma: 11 ##### Re: 2.6 Q22
« Reply #1 on: November 22, 2018, 02:12:09 PM »
I don't understand your question.
Isn't it just substituting in the number?

#### Min Gyu Woo

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« Reply #2 on: November 22, 2018, 02:32:44 PM »
The steps of getting to that final equation. Not the formula.

#### ruienlin

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« Reply #3 on: November 22, 2018, 02:33:25 PM »
Hi Kathy, here is my solution to (a):
For Question(a),$$\int_0^\infty \frac{d_x}{8+x^3}=\frac{1}{8} \int_0^\infty \frac{d_x}{1+\frac{x^3}{8}}$$
$$\int_0^\infty \frac{d_x}{1+\frac{x^3}{8}}=\int_0^\infty \frac{d_x}{1+ ( \frac{x}{2})^3}$$
Change variable $x/2$ to $t$ and apply (9), then it becomes
$$2\int_0^\infty \frac{d_t}{1+ t^3} = \frac{2}{3}\frac{\pi}{\sin(\frac{\pi}{3})}=\frac{4\sqrt{3}\pi}{9}$$
so,$$\int_0^\infty \frac{d_x}{8+x^3}=\frac{\sqrt{3}\pi}{18}$$

#### Victor Ivrii ##### Re: 2.6 Q22
« Reply #4 on: November 22, 2018, 02:37:01 PM »
consider (c), which is the most general, and consider a "pizza contour" $\gamma$ with an angle $\theta$ such that $e^{i\beta\theta}=1$ (it may be greater than $2\pi$, but it really does not matter) and also if $-1<\gamma <0$ you should cut a small piece of the radius $\varepsilon$. Consider
$$\int_\gamma \frac{z^\gamma\,dz}{1+z^\beta}.$$
What are singularities inside? Calculate the residue (or residues).

Prove that for $R\to \infty$ the integral over big arc tends to $0$
Prove that for $\varepsilon\to 0$ the integral over big arc tends to $0$

Express integrals over straight segments via
$$\int_\varepsilon ^R \frac{x^\gamma\,dx}{1+x^\beta}.$$
Then after taking the limits  you'll be able to find integral in question.

There are Examples in the section, using exactly the same method.

#### yuruoyun

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« Reply #5 on: November 22, 2018, 02:41:00 PM »
For Question(b),$$\int_0^\infty \frac{xd_x}{x^4+16} = \frac{1}{16} \int_0^\infty \frac{xd_x}{\frac{x^4}{16}+1}$$
Change variable $x/2$ to $t$ and apply 8, then it becomes
$$2\int_0^\infty \frac{2td_t}{1+ t^4} = 4\int_0^\infty \frac{td_t}{1+ t^4}$$
$\beta = 4, \alpha\beta-1 = 1$, so $\alpha = \frac{1}{2}$
$$\int_0^\infty \frac{xd_x}{\frac{x^4}{16}+1}=\frac{4}{4}\frac{\pi}{\sin(\frac{\pi}{2})}=\pi$$
so$$\int_0^\infty \frac{xd_x}{x^4+16} = \frac{\pi}{16}$$
Question(c) is the same following the same logic, with $0\leq\gamma\lt\beta-1$
$$\int_0^\infty \frac{x^\gamma d_x}{1+x^\beta}$$
$$\alpha\beta-1=\gamma$$
$$\alpha=\frac{\gamma+1}{\beta}, \frac{1}{\beta}\le\frac{\gamma+1}{\beta}\lt1$$
$$\int_0^\infty \frac{x^\gamma d_x}{1+x^\beta} = \frac{\pi}{\beta\sin(\frac{(\gamma+1)\pi}{\beta})}$$

#### Victor Ivrii ##### Re: 2.6 Q22
« Reply #6 on: November 22, 2018, 03:59:18 PM »
You should not use ready formulae but to derive the answer using residue theorem.

#### Ende Jin

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• Karma: 11 ##### Re: 2.6 Q22
« Reply #7 on: November 23, 2018, 07:25:37 PM »
Why "greater than $2 \pi$" does not matter?
I see that you didn't emphasize $\beta$ to be a natural number, which is a bit weird because I don't even know how many singularities there are.

#### Victor Ivrii ##### Re: 2.6 Q22
« Reply #8 on: November 24, 2018, 04:24:02 AM »
Why "greater than $2 \pi$" does not matter?
I see that you didn't emphasize $\beta$ to be a natural number, which is a bit weird because I don't even know how many singularities there are.
We can reduce any $\beta>0$ to $\beta=2$ by substitution $x=t^{2/\beta}$; then $\gamma$ is replaced by $\delta=2(\gamma +1)/\beta -1$ and if $-1<\gamma<\beta-1$ (conditions needed to have convergency at $x=0$ and $x=\infty$ respectively), then $-1<\delta<1$.

We need only $\beta >0$; there will be just one singularity in the sector $0<\arg (z)< 2\pi/\beta$; namely as $z=e^{i\pi/beta}$.
« Last Edit: November 27, 2018, 01:53:01 PM by Victor Ivrii »

#### terryzhang

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« Reply #9 on: November 27, 2018, 11:59:41 AM »
Hi, Professor
If we do not use the ready formula for (c), does it mean that we need to prove this formula?
« Last Edit: November 27, 2018, 12:05:58 PM by terryzhang »

#### Victor Ivrii ##### Re: 2.6 Q22
« Reply #10 on: November 27, 2018, 01:54:39 PM »
If we do not use the ready formula for (c), does it mean that we need to prove this formula?
Yes, either in the general case, or in the special case of the given problem.