How to solve this definite integral $int_0^pi frac{cos^9(x)}{sin^3(x)+cos^3(x)}dx$
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
|
show 3 more comments
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
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
|
show 3 more comments
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
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
integration definite-integrals
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
|
show 3 more comments
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
|
show 3 more comments
1 Answer
1
active
oldest
votes
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,
$$
I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
\ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
\ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
\ = frac{5pi}{16}
$$
add a comment |
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1 Answer
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1 Answer
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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,
$$
I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
\ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
\ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
\ = frac{5pi}{16}
$$
add a comment |
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,
$$
I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
\ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
\ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
\ = frac{5pi}{16}
$$
add a comment |
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,
$$
I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
\ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
\ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
\ = frac{5pi}{16}
$$
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,
$$
I_1 + I_2 = text{p. v.} int_0^pi frac{cos^9(x)}{cos^3(x)+sin^3(x)} {rm d}x
\ = int_0^fracpi2 sin^6(x) + cos^6(x) {rm d}x
\ = int_0^fracpi2 frac58 + frac38 cos(4x) {rm d}x
\ = frac{5pi}{16}
$$
edited Dec 26 at 15:32
answered Dec 20 at 18:25
Mint
4881416
4881416
add a comment |
add a comment |
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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