Solving Integration: No Numerical Method Needed, Answer is 1

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In summary: I spent awhile on this and eventually asked Mathematica for help in calculating the integral. It confirmed the answer as \approx 1.21883, and gave that\int^{\pi }_0{{cos}^2\left(\frac{\pi }{2}{\cos \left(x\right)\ }\right){\csc \left(x\right)\ }dx=\frac{1}{2}\left(-Ci(2\pi \right)+\gamma +{\rm ln}(2\pi ))}.
  • #1
yungman
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Please tell me what did I do wrong on this integration. The book claimed this can only be solved in numerical method and the answer is 1.218.

[tex]\int _0^{\pi} \frac {\cos ^2\left [\frac {\pi} 2 \;\cos\;\theta\right]}{\sin \theta}\;d\;\theta [/tex]

[tex]\hbox { Let }\; u=\frac {\pi} 2\; \cos \theta\;\Rightarrow\; d\theta =-\frac{2\;d\;u}{\pi\;\sin\;\theta}\; \Rightarrow\;\int _0^{\pi} \frac {\cos ^2\left [\frac {\pi} 2 \;\cos\;\theta\right]}{\sin \theta}\;d\;\theta \;=\; -\frac 2 {\pi}\int \cos^2\;u \;d\;u\;=-\frac 1 {\pi} \left[ \int d\;u \;+\; \int \cos(2u) d\;u \right ][/tex]

[tex] = -\left [\frac { \frac {\pi} 2 \cos \theta}{\pi}\right]_0^{\pi} \;-\;\frac 1 {2\pi} \int \cos v \;dv \;\;\;\;\;\;\hbox { where }\;\; \;v=2u \;\hbox { and }\;d\;u=\frac {d\;v} 2 [/tex]

[tex]= 1 -\left[\frac { \sin (2\frac {\pi} 2 \;\cos \theta)}{2\pi}\right]^{\pi}_0 \;= 1 [/tex]

I solve this without using numerical method and the answer is 1 instead of 1.218. Who is right?
 
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  • #2
on your first u-substitution u=pi/2*cos(x)
your du=sin(x)dx
your du is not [itex] du= \frac{1}{sin(x)} [/itex]
you can't get rid of that sine in the bottom with that u substitution
 
  • #3
cragar said:
on your first u-substitution u=pi/2*cos(x)
your du=sin(x)dx
your du is not [itex] du= \frac{1}{sin(x)} [/itex]
you can't get rid of that sine in the bottom with that u substitution
I really don't get it. Please explain more.Thanks

Alan
 
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  • #4
yes that's what you would have. This integral looks tricky if its even doable, I tried some stuff using trig identities and stuff and even thought about using Eulers formula .
 
  • #5
yungman said:
Do you mean after the substitution, it is [itex]cos^2(\frac u {sin \theta}) [/itex] instead. So I cannot cancel the one outside?

After the substitution the integrand should become

[itex] -\frac{2}{\pi}\frac{cos^2 u}{sin^2 \theta} [/itex]
 
  • #6
I spent awhile on this and eventually asked Mathematica for help in calculating the integral. It confirmed the answer as [itex]\approx 1.21883[/itex], and gave that
[itex]\int^{\pi }_0{{cos}^2\left(\frac{\pi }{2}{\cos \left(x\right)\ }\right){\csc \left(x\right)\ }dx=\frac{1}{2}\left(-Ci(2\pi \right)+\gamma +{\rm ln}(2\pi ))}.[/itex]
Where Ci(x) is the cosine integral (http://mathworld.wolfram.com/CosineIntegral.html) and [itex]\gamma[/itex] is the Euler-Mascheroni constant (http://mathworld.wolfram.com/Euler-MascheroniConstant.html), which would seem to confirm this as a non-elementary integral. I don't even think it's possible to solve without the aid of a very comprehensive set of integration tables. =\
 
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  • #7
Isak BM said:
After the substitution the integrand should become

[itex] -\frac{2}{\pi}\frac{cos^2 u}{sin^2 \theta} [/itex]

Thanks, I was so blind! No wonder! It was late last night. I don't know why I tend to make this kind of stupid mistake all the times, it is so obvious that I miss it. That's the reason I never get 100 in my ODE class, always 96 97, always have one of these mistakes to take off a few points! Kicker is I still did not see it after your first reply...and I did went through the whole thing!

Thanks.
 
  • #8
akaritakai said:
I spent awhile on this and eventually asked Mathematica for help in calculating the integral. It confirmed the answer as [itex]\approx 1.21883[/itex], and gave that
[itex]\int^{\pi }_0{{cos}^2\left(\frac{\pi }{2}{\cos \left(x\right)\ }\right){\csc \left(x\right)\ }dx=\frac{1}{2}\left(-Ci(2\pi \right)+\gamma +{\rm ln}(2\pi ))}.[/itex]
Where Ci(x) is the cosine integral (http://mathworld.wolfram.com/CosineIntegral.html) and [itex]\gamma[/itex] is the Euler-Mascheroni constant (http://mathworld.wolfram.com/Euler-MascheroniConstant.html), which would seem to confirm this as a non-elementary integral. I don't even think it's possible to solve without the aid of a very comprehensive set of integration tables. =\

Thanks.
 
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FAQ: Solving Integration: No Numerical Method Needed, Answer is 1

What is integration?

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