How to solve this definite integral $int_0^pi frac{cos^9(x)}{sin^3(x)+cos^3(x)}dx$












3














I'm having trouble evaluating the following integral:
$$
int^pi_0 frac{cos^9(x)}{sin^3(x)+cos^3(x)}dx
$$



I tried to convert it into an algebraic function by multiplying the numerator and denominator by $sec^{11}(x)$ and substituting $tan(x)=t$ as



$$
int^pi_0 frac{cos^9(x)cdot sec^{11}(x)}{(sin^3(x)+cos^3(x))cdot sec^{11}(x)}dx
$$



$$
int^pi_0 frac{sec^2(x)}{(tan^3(x)+1)cdot (tan^2(x)+1)^4}dx
$$



Substituting $tan(x)=t$,



$$
int^0_0 frac{dt}{(t^3+1)cdot (t^2+1)^4}
$$



But now both upper and lower limit become $0$ so apparently this is not the right approach, so how do I go about solving it?










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  • 10




    Before you do anything, you need to decide what you want to do with the singularity at $x = frac{3pi}{4}$.
    – achille hui
    Jan 16 '16 at 19:20






  • 2




    One option is compute the Cauchy principal value instead. Another option is lift the integral to a contour integral in complex plane and deform the contour around the pole at $x = frac{3pi}{4}$. If this is the first time you deal with this sort of integral, you should follow the Cauchy principle value route. You essentially need to split the integral to 2 regions $[0,frac{3pi}{4}-epsilon], [frac{3pi}{4}+epsilon,pi]$ and take the limit $epsilon to 0^{+}$ at the end.
    – achille hui
    Jan 16 '16 at 19:32






  • 4




    Sure the integral is not on $[0,pi/2]$?
    – Did
    Jan 16 '16 at 20:19






  • 1




    "no harm in learning something new" Sure, and this is related how?
    – Did
    Jan 17 '16 at 10:34






  • 1




    When you do your substitutions, the functions in your substitutions must be continuous over the required interval. In this case, break it up into $(0, frac {pi}{2}), (frac {pi}{2}, pi)$ and you should be fine.
    – Doug M
    Dec 20 at 18:42


















3














I'm having trouble evaluating the following integral:
$$
int^pi_0 frac{cos^9(x)}{sin^3(x)+cos^3(x)}dx
$$



I tried to convert it into an algebraic function by multiplying the numerator and denominator by $sec^{11}(x)$ and substituting $tan(x)=t$ as



$$
int^pi_0 frac{cos^9(x)cdot sec^{11}(x)}{(sin^3(x)+cos^3(x))cdot sec^{11}(x)}dx
$$



$$
int^pi_0 frac{sec^2(x)}{(tan^3(x)+1)cdot (tan^2(x)+1)^4}dx
$$



Substituting $tan(x)=t$,



$$
int^0_0 frac{dt}{(t^3+1)cdot (t^2+1)^4}
$$



But now both upper and lower limit become $0$ so apparently this is not the right approach, so how do I go about solving it?










share|cite|improve this question




















  • 10




    Before you do anything, you need to decide what you want to do with the singularity at $x = frac{3pi}{4}$.
    – achille hui
    Jan 16 '16 at 19:20






  • 2




    One option is compute the Cauchy principal value instead. Another option is lift the integral to a contour integral in complex plane and deform the contour around the pole at $x = frac{3pi}{4}$. If this is the first time you deal with this sort of integral, you should follow the Cauchy principle value route. You essentially need to split the integral to 2 regions $[0,frac{3pi}{4}-epsilon], [frac{3pi}{4}+epsilon,pi]$ and take the limit $epsilon to 0^{+}$ at the end.
    – achille hui
    Jan 16 '16 at 19:32






  • 4




    Sure the integral is not on $[0,pi/2]$?
    – Did
    Jan 16 '16 at 20:19






  • 1




    "no harm in learning something new" Sure, and this is related how?
    – Did
    Jan 17 '16 at 10:34






  • 1




    When you do your substitutions, the functions in your substitutions must be continuous over the required interval. In this case, break it up into $(0, frac {pi}{2}), (frac {pi}{2}, pi)$ and you should be fine.
    – Doug M
    Dec 20 at 18:42
















3












3








3


1





I'm having trouble evaluating the following integral:
$$
int^pi_0 frac{cos^9(x)}{sin^3(x)+cos^3(x)}dx
$$



I tried to convert it into an algebraic function by multiplying the numerator and denominator by $sec^{11}(x)$ and substituting $tan(x)=t$ as



$$
int^pi_0 frac{cos^9(x)cdot sec^{11}(x)}{(sin^3(x)+cos^3(x))cdot sec^{11}(x)}dx
$$



$$
int^pi_0 frac{sec^2(x)}{(tan^3(x)+1)cdot (tan^2(x)+1)^4}dx
$$



Substituting $tan(x)=t$,



$$
int^0_0 frac{dt}{(t^3+1)cdot (t^2+1)^4}
$$



But now both upper and lower limit become $0$ so apparently this is not the right approach, so how do I go about solving it?










share|cite|improve this question















I'm having trouble evaluating the following integral:
$$
int^pi_0 frac{cos^9(x)}{sin^3(x)+cos^3(x)}dx
$$



I tried to convert it into an algebraic function by multiplying the numerator and denominator by $sec^{11}(x)$ and substituting $tan(x)=t$ as



$$
int^pi_0 frac{cos^9(x)cdot sec^{11}(x)}{(sin^3(x)+cos^3(x))cdot sec^{11}(x)}dx
$$



$$
int^pi_0 frac{sec^2(x)}{(tan^3(x)+1)cdot (tan^2(x)+1)^4}dx
$$



Substituting $tan(x)=t$,



$$
int^0_0 frac{dt}{(t^3+1)cdot (t^2+1)^4}
$$



But now both upper and lower limit become $0$ so apparently this is not the right approach, so how do I go about solving it?







integration definite-integrals






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share|cite|improve this question













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edited May 27 '16 at 1:48









Martin Sleziak

44.7k7115270




44.7k7115270










asked Jan 16 '16 at 19:13









Apoorv

1436




1436








  • 10




    Before you do anything, you need to decide what you want to do with the singularity at $x = frac{3pi}{4}$.
    – achille hui
    Jan 16 '16 at 19:20






  • 2




    One option is compute the Cauchy principal value instead. Another option is lift the integral to a contour integral in complex plane and deform the contour around the pole at $x = frac{3pi}{4}$. If this is the first time you deal with this sort of integral, you should follow the Cauchy principle value route. You essentially need to split the integral to 2 regions $[0,frac{3pi}{4}-epsilon], [frac{3pi}{4}+epsilon,pi]$ and take the limit $epsilon to 0^{+}$ at the end.
    – achille hui
    Jan 16 '16 at 19:32






  • 4




    Sure the integral is not on $[0,pi/2]$?
    – Did
    Jan 16 '16 at 20:19






  • 1




    "no harm in learning something new" Sure, and this is related how?
    – Did
    Jan 17 '16 at 10:34






  • 1




    When you do your substitutions, the functions in your substitutions must be continuous over the required interval. In this case, break it up into $(0, frac {pi}{2}), (frac {pi}{2}, pi)$ and you should be fine.
    – Doug M
    Dec 20 at 18:42
















  • 10




    Before you do anything, you need to decide what you want to do with the singularity at $x = frac{3pi}{4}$.
    – achille hui
    Jan 16 '16 at 19:20






  • 2




    One option is compute the Cauchy principal value instead. Another option is lift the integral to a contour integral in complex plane and deform the contour around the pole at $x = frac{3pi}{4}$. If this is the first time you deal with this sort of integral, you should follow the Cauchy principle value route. You essentially need to split the integral to 2 regions $[0,frac{3pi}{4}-epsilon], [frac{3pi}{4}+epsilon,pi]$ and take the limit $epsilon to 0^{+}$ at the end.
    – achille hui
    Jan 16 '16 at 19:32






  • 4




    Sure the integral is not on $[0,pi/2]$?
    – Did
    Jan 16 '16 at 20:19






  • 1




    "no harm in learning something new" Sure, and this is related how?
    – Did
    Jan 17 '16 at 10:34






  • 1




    When you do your substitutions, the functions in your substitutions must be continuous over the required interval. In this case, break it up into $(0, frac {pi}{2}), (frac {pi}{2}, pi)$ and you should be fine.
    – Doug M
    Dec 20 at 18:42










10




10




Before you do anything, you need to decide what you want to do with the singularity at $x = frac{3pi}{4}$.
– achille hui
Jan 16 '16 at 19:20




Before you do anything, you need to decide what you want to do with the singularity at $x = frac{3pi}{4}$.
– achille hui
Jan 16 '16 at 19:20




2




2




One option is compute the Cauchy principal value instead. Another option is lift the integral to a contour integral in complex plane and deform the contour around the pole at $x = frac{3pi}{4}$. If this is the first time you deal with this sort of integral, you should follow the Cauchy principle value route. You essentially need to split the integral to 2 regions $[0,frac{3pi}{4}-epsilon], [frac{3pi}{4}+epsilon,pi]$ and take the limit $epsilon to 0^{+}$ at the end.
– achille hui
Jan 16 '16 at 19:32




One option is compute the Cauchy principal value instead. Another option is lift the integral to a contour integral in complex plane and deform the contour around the pole at $x = frac{3pi}{4}$. If this is the first time you deal with this sort of integral, you should follow the Cauchy principle value route. You essentially need to split the integral to 2 regions $[0,frac{3pi}{4}-epsilon], [frac{3pi}{4}+epsilon,pi]$ and take the limit $epsilon to 0^{+}$ at the end.
– achille hui
Jan 16 '16 at 19:32




4




4




Sure the integral is not on $[0,pi/2]$?
– Did
Jan 16 '16 at 20:19




Sure the integral is not on $[0,pi/2]$?
– Did
Jan 16 '16 at 20:19




1




1




"no harm in learning something new" Sure, and this is related how?
– Did
Jan 17 '16 at 10:34




"no harm in learning something new" Sure, and this is related how?
– Did
Jan 17 '16 at 10:34




1




1




When you do your substitutions, the functions in your substitutions must be continuous over the required interval. In this case, break it up into $(0, frac {pi}{2}), (frac {pi}{2}, pi)$ and you should be fine.
– Doug M
Dec 20 at 18:42






When you do your substitutions, the functions in your substitutions must be continuous over the required interval. In this case, break it up into $(0, frac {pi}{2}), (frac {pi}{2}, pi)$ and you should be fine.
– Doug M
Dec 20 at 18:42












1 Answer
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1














Assuming the usage of the Cauchy principle value, let's define
$$ I_1 := int_0^fracpi2 frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
\ I_2 := text{p. v.} int_fracpi2^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
$$



For $I_1$, by making a substitution and adding it to itself,
$$
I_1 = int_0^fracpi2 frac{sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
\ = frac12 int_0^fracpi2 frac{cos^9(x) +sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
\ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) - sin^3(x)cos^3(x) right) {rm d}x
$$



For $I_2$, by making some substitutions,
$$
I_2 = text{p. v.} int_0^fracpi2 frac{sin^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
\ = text{p. v.} int_0^fracpi2 frac{-cos^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
\ = frac12 text{p. v.} int_0^fracpi2 frac{sin^9(x)-cos^9(x)}{sin^3(x)-cos^3(x)}
\ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) + sin^3(x)cos^3(x) right) {rm d}x
$$



Therefore,



$$
I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
\ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
\ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
\ = frac{5pi}{16}
$$






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    Assuming the usage of the Cauchy principle value, let's define
    $$ I_1 := int_0^fracpi2 frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
    \ I_2 := text{p. v.} int_fracpi2^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
    $$



    For $I_1$, by making a substitution and adding it to itself,
    $$
    I_1 = int_0^fracpi2 frac{sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
    \ = frac12 int_0^fracpi2 frac{cos^9(x) +sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
    \ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) - sin^3(x)cos^3(x) right) {rm d}x
    $$



    For $I_2$, by making some substitutions,
    $$
    I_2 = text{p. v.} int_0^fracpi2 frac{sin^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
    \ = text{p. v.} int_0^fracpi2 frac{-cos^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
    \ = frac12 text{p. v.} int_0^fracpi2 frac{sin^9(x)-cos^9(x)}{sin^3(x)-cos^3(x)}
    \ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) + sin^3(x)cos^3(x) right) {rm d}x
    $$



    Therefore,



    $$
    I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
    \ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
    \ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
    \ = frac{5pi}{16}
    $$






    share|cite|improve this answer




























      1














      Assuming the usage of the Cauchy principle value, let's define
      $$ I_1 := int_0^fracpi2 frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
      \ I_2 := text{p. v.} int_fracpi2^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
      $$



      For $I_1$, by making a substitution and adding it to itself,
      $$
      I_1 = int_0^fracpi2 frac{sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
      \ = frac12 int_0^fracpi2 frac{cos^9(x) +sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
      \ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) - sin^3(x)cos^3(x) right) {rm d}x
      $$



      For $I_2$, by making some substitutions,
      $$
      I_2 = text{p. v.} int_0^fracpi2 frac{sin^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
      \ = text{p. v.} int_0^fracpi2 frac{-cos^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
      \ = frac12 text{p. v.} int_0^fracpi2 frac{sin^9(x)-cos^9(x)}{sin^3(x)-cos^3(x)}
      \ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) + sin^3(x)cos^3(x) right) {rm d}x
      $$



      Therefore,



      $$
      I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
      \ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
      \ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
      \ = frac{5pi}{16}
      $$






      share|cite|improve this answer


























        1












        1








        1






        Assuming the usage of the Cauchy principle value, let's define
        $$ I_1 := int_0^fracpi2 frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        \ I_2 := text{p. v.} int_fracpi2^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        $$



        For $I_1$, by making a substitution and adding it to itself,
        $$
        I_1 = int_0^fracpi2 frac{sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        \ = frac12 int_0^fracpi2 frac{cos^9(x) +sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        \ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) - sin^3(x)cos^3(x) right) {rm d}x
        $$



        For $I_2$, by making some substitutions,
        $$
        I_2 = text{p. v.} int_0^fracpi2 frac{sin^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
        \ = text{p. v.} int_0^fracpi2 frac{-cos^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
        \ = frac12 text{p. v.} int_0^fracpi2 frac{sin^9(x)-cos^9(x)}{sin^3(x)-cos^3(x)}
        \ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) + sin^3(x)cos^3(x) right) {rm d}x
        $$



        Therefore,



        $$
        I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        \ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
        \ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
        \ = frac{5pi}{16}
        $$






        share|cite|improve this answer














        Assuming the usage of the Cauchy principle value, let's define
        $$ I_1 := int_0^fracpi2 frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        \ I_2 := text{p. v.} int_fracpi2^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        $$



        For $I_1$, by making a substitution and adding it to itself,
        $$
        I_1 = int_0^fracpi2 frac{sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        \ = frac12 int_0^fracpi2 frac{cos^9(x) +sin^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        \ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) - sin^3(x)cos^3(x) right) {rm d}x
        $$



        For $I_2$, by making some substitutions,
        $$
        I_2 = text{p. v.} int_0^fracpi2 frac{sin^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
        \ = text{p. v.} int_0^fracpi2 frac{-cos^9(x)}{sin^3(x)-cos^3(x)} {rm d}x
        \ = frac12 text{p. v.} int_0^fracpi2 frac{sin^9(x)-cos^9(x)}{sin^3(x)-cos^3(x)}
        \ = frac12 int_0^fracpi2 left( sin^6(x) + cos^6(x) + sin^3(x)cos^3(x) right) {rm d}x
        $$



        Therefore,



        $$
        I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
        \ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
        \ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
        \ = frac{5pi}{16}
        $$







        share|cite|improve this answer














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        edited Dec 26 at 15:32

























        answered Dec 20 at 18:25









        Mint

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        4881416






























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