Is there a clever way to compute $int limits_a^b sqrt[3](b-x)(x-a)^2 dx$
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I'm wondering how to compute $int limits_a^b sqrt[3](b-x)(x-a)^2 dx$. I tried doing Chebyshe[o]v's substitution to first compute the antiderivative which led me to $int fracw^3(w^3+1)^3 dw$ what is quite unpleasant as Wolframalpha says.
Thanks.
integration radicals
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up vote
2
down vote
favorite
I'm wondering how to compute $int limits_a^b sqrt[3](b-x)(x-a)^2 dx$. I tried doing Chebyshe[o]v's substitution to first compute the antiderivative which led me to $int fracw^3(w^3+1)^3 dw$ what is quite unpleasant as Wolframalpha says.
Thanks.
integration radicals
$$x=acos^2t+bsin^2t$$
– lab bhattacharjee
Jul 25 at 16:41
@labbhattacharjee And then what?
– Mark Viola
Jul 25 at 17:27
add a comment |Â
up vote
2
down vote
favorite
up vote
2
down vote
favorite
I'm wondering how to compute $int limits_a^b sqrt[3](b-x)(x-a)^2 dx$. I tried doing Chebyshe[o]v's substitution to first compute the antiderivative which led me to $int fracw^3(w^3+1)^3 dw$ what is quite unpleasant as Wolframalpha says.
Thanks.
integration radicals
I'm wondering how to compute $int limits_a^b sqrt[3](b-x)(x-a)^2 dx$. I tried doing Chebyshe[o]v's substitution to first compute the antiderivative which led me to $int fracw^3(w^3+1)^3 dw$ what is quite unpleasant as Wolframalpha says.
Thanks.
integration radicals
asked Jul 25 at 16:26
Nicholas S
17010
17010
$$x=acos^2t+bsin^2t$$
– lab bhattacharjee
Jul 25 at 16:41
@labbhattacharjee And then what?
– Mark Viola
Jul 25 at 17:27
add a comment |Â
$$x=acos^2t+bsin^2t$$
– lab bhattacharjee
Jul 25 at 16:41
@labbhattacharjee And then what?
– Mark Viola
Jul 25 at 17:27
$$x=acos^2t+bsin^2t$$
– lab bhattacharjee
Jul 25 at 16:41
$$x=acos^2t+bsin^2t$$
– lab bhattacharjee
Jul 25 at 16:41
@labbhattacharjee And then what?
– Mark Viola
Jul 25 at 17:27
@labbhattacharjee And then what?
– Mark Viola
Jul 25 at 17:27
add a comment |Â
2 Answers
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up vote
2
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accepted
It's better to do this without trying to find an antiderivative. Substituting $x=a+(b-a)y$, we find that $x-a = (b-a)y$ and $b-x = (b-a)(1-y)$, so the integral reduces to
$$ (b-a)^2 int_0^1 y^2/3 (1-y)^1/3 , dy. $$
The remaining integral is $B(frac53,frac43) = 2pi/3^5/2 $ using properties of the Beta- and Gamma-functions. An elementary way of computing this Beta-function is to set $y=1/(1+u)$, which gives
$$ int_0^infty fracy^1/3(1+y)^3 , dy. $$
This can be calculated by differentiating the integral
$$ int_0^infty fracy^s-1a+y , dy = a^s-1picscpi s $$
a couple of times.
add a comment |Â
up vote
0
down vote
Thanks @Chappers for an idea. Here is the way of computing the product they was talking about:
We will require two basic properties. The first one is the Euler reflection formula:
$$Gamma(1-z)Gamma(z) = fracpisin z pi$$
The second is $Gamma(z+1)=zGamma(z)$. So,
$$Gammaleft(frac53right) Gammaleft(-frac23 right) = fracpisin left(-frac23 piright)$$
$$Gammaleft(frac43right) Gammaleft(-frac13 right) = fracpisin left(-frac13 piright)$$
We multiply:
$$Gamma(5/3)Gamma(-2/3)Gamma(4/3)Gamma(-1/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)$$
$$Gamma(5/3)Gamma(4/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)Gamma(-2/3)Gamma(-1/3)$$
$$Gammaleft(frac13 right)=Gammaleft(-frac23+1 right)=-frac23Gammaleft(-frac23 right)$$ $$Gammaleft(frac23 right)=Gammaleft(-frac13+1 right)=-frac13Gammaleft(-frac13 right)$$
Hence
$$Gammaleft(-frac13 right) = -3Gammaleft(frac23 right) text ø Gammaleft(-frac23 right) = -frac32Gammaleft(frac13 right)$$
Finally,
$$Gammaleft(frac53right)Gammaleft(frac43right) =displaystylefracpi^2sin left(-frac23 piright) cdot sin left(-frac13 piright)cdot -frac32 Gammaleft(frac13right) cdot(-3)Gammaleft(frac23right)=$$
$$=frac29cdotfracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)underbraceGammaleft(1/3right) Gammaleft(2/3right)_displaystylefracpisin(1/3pi)=$$
$$frac29cdotfracpi cdot sin (1/3pi)sin(-2/3pi)cdot sin(-1/3 pi)= frac4pi9sqrt3$$
So:
$$mathrmBleft(frac53,frac43 right) = fracGamma(5/3) Gamma(4/3)Gamma(3) = fracGamma(5/3) Gamma(4/3)(3-1)!=$$
$$= fracGamma(5/3) Gamma(4/3)2=frac2pi9sqrt3$$
In conclusion we obtain:
$$boxeddisplaystyleint limits_a^b sqrt[3](b-x)(x-a)^2 dx = (b-a)^2frac2pi9sqrt3$$
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
accepted
It's better to do this without trying to find an antiderivative. Substituting $x=a+(b-a)y$, we find that $x-a = (b-a)y$ and $b-x = (b-a)(1-y)$, so the integral reduces to
$$ (b-a)^2 int_0^1 y^2/3 (1-y)^1/3 , dy. $$
The remaining integral is $B(frac53,frac43) = 2pi/3^5/2 $ using properties of the Beta- and Gamma-functions. An elementary way of computing this Beta-function is to set $y=1/(1+u)$, which gives
$$ int_0^infty fracy^1/3(1+y)^3 , dy. $$
This can be calculated by differentiating the integral
$$ int_0^infty fracy^s-1a+y , dy = a^s-1picscpi s $$
a couple of times.
add a comment |Â
up vote
2
down vote
accepted
It's better to do this without trying to find an antiderivative. Substituting $x=a+(b-a)y$, we find that $x-a = (b-a)y$ and $b-x = (b-a)(1-y)$, so the integral reduces to
$$ (b-a)^2 int_0^1 y^2/3 (1-y)^1/3 , dy. $$
The remaining integral is $B(frac53,frac43) = 2pi/3^5/2 $ using properties of the Beta- and Gamma-functions. An elementary way of computing this Beta-function is to set $y=1/(1+u)$, which gives
$$ int_0^infty fracy^1/3(1+y)^3 , dy. $$
This can be calculated by differentiating the integral
$$ int_0^infty fracy^s-1a+y , dy = a^s-1picscpi s $$
a couple of times.
add a comment |Â
up vote
2
down vote
accepted
up vote
2
down vote
accepted
It's better to do this without trying to find an antiderivative. Substituting $x=a+(b-a)y$, we find that $x-a = (b-a)y$ and $b-x = (b-a)(1-y)$, so the integral reduces to
$$ (b-a)^2 int_0^1 y^2/3 (1-y)^1/3 , dy. $$
The remaining integral is $B(frac53,frac43) = 2pi/3^5/2 $ using properties of the Beta- and Gamma-functions. An elementary way of computing this Beta-function is to set $y=1/(1+u)$, which gives
$$ int_0^infty fracy^1/3(1+y)^3 , dy. $$
This can be calculated by differentiating the integral
$$ int_0^infty fracy^s-1a+y , dy = a^s-1picscpi s $$
a couple of times.
It's better to do this without trying to find an antiderivative. Substituting $x=a+(b-a)y$, we find that $x-a = (b-a)y$ and $b-x = (b-a)(1-y)$, so the integral reduces to
$$ (b-a)^2 int_0^1 y^2/3 (1-y)^1/3 , dy. $$
The remaining integral is $B(frac53,frac43) = 2pi/3^5/2 $ using properties of the Beta- and Gamma-functions. An elementary way of computing this Beta-function is to set $y=1/(1+u)$, which gives
$$ int_0^infty fracy^1/3(1+y)^3 , dy. $$
This can be calculated by differentiating the integral
$$ int_0^infty fracy^s-1a+y , dy = a^s-1picscpi s $$
a couple of times.
answered Jul 25 at 16:47


Chappers
55k74190
55k74190
add a comment |Â
add a comment |Â
up vote
0
down vote
Thanks @Chappers for an idea. Here is the way of computing the product they was talking about:
We will require two basic properties. The first one is the Euler reflection formula:
$$Gamma(1-z)Gamma(z) = fracpisin z pi$$
The second is $Gamma(z+1)=zGamma(z)$. So,
$$Gammaleft(frac53right) Gammaleft(-frac23 right) = fracpisin left(-frac23 piright)$$
$$Gammaleft(frac43right) Gammaleft(-frac13 right) = fracpisin left(-frac13 piright)$$
We multiply:
$$Gamma(5/3)Gamma(-2/3)Gamma(4/3)Gamma(-1/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)$$
$$Gamma(5/3)Gamma(4/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)Gamma(-2/3)Gamma(-1/3)$$
$$Gammaleft(frac13 right)=Gammaleft(-frac23+1 right)=-frac23Gammaleft(-frac23 right)$$ $$Gammaleft(frac23 right)=Gammaleft(-frac13+1 right)=-frac13Gammaleft(-frac13 right)$$
Hence
$$Gammaleft(-frac13 right) = -3Gammaleft(frac23 right) text ø Gammaleft(-frac23 right) = -frac32Gammaleft(frac13 right)$$
Finally,
$$Gammaleft(frac53right)Gammaleft(frac43right) =displaystylefracpi^2sin left(-frac23 piright) cdot sin left(-frac13 piright)cdot -frac32 Gammaleft(frac13right) cdot(-3)Gammaleft(frac23right)=$$
$$=frac29cdotfracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)underbraceGammaleft(1/3right) Gammaleft(2/3right)_displaystylefracpisin(1/3pi)=$$
$$frac29cdotfracpi cdot sin (1/3pi)sin(-2/3pi)cdot sin(-1/3 pi)= frac4pi9sqrt3$$
So:
$$mathrmBleft(frac53,frac43 right) = fracGamma(5/3) Gamma(4/3)Gamma(3) = fracGamma(5/3) Gamma(4/3)(3-1)!=$$
$$= fracGamma(5/3) Gamma(4/3)2=frac2pi9sqrt3$$
In conclusion we obtain:
$$boxeddisplaystyleint limits_a^b sqrt[3](b-x)(x-a)^2 dx = (b-a)^2frac2pi9sqrt3$$
add a comment |Â
up vote
0
down vote
Thanks @Chappers for an idea. Here is the way of computing the product they was talking about:
We will require two basic properties. The first one is the Euler reflection formula:
$$Gamma(1-z)Gamma(z) = fracpisin z pi$$
The second is $Gamma(z+1)=zGamma(z)$. So,
$$Gammaleft(frac53right) Gammaleft(-frac23 right) = fracpisin left(-frac23 piright)$$
$$Gammaleft(frac43right) Gammaleft(-frac13 right) = fracpisin left(-frac13 piright)$$
We multiply:
$$Gamma(5/3)Gamma(-2/3)Gamma(4/3)Gamma(-1/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)$$
$$Gamma(5/3)Gamma(4/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)Gamma(-2/3)Gamma(-1/3)$$
$$Gammaleft(frac13 right)=Gammaleft(-frac23+1 right)=-frac23Gammaleft(-frac23 right)$$ $$Gammaleft(frac23 right)=Gammaleft(-frac13+1 right)=-frac13Gammaleft(-frac13 right)$$
Hence
$$Gammaleft(-frac13 right) = -3Gammaleft(frac23 right) text ø Gammaleft(-frac23 right) = -frac32Gammaleft(frac13 right)$$
Finally,
$$Gammaleft(frac53right)Gammaleft(frac43right) =displaystylefracpi^2sin left(-frac23 piright) cdot sin left(-frac13 piright)cdot -frac32 Gammaleft(frac13right) cdot(-3)Gammaleft(frac23right)=$$
$$=frac29cdotfracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)underbraceGammaleft(1/3right) Gammaleft(2/3right)_displaystylefracpisin(1/3pi)=$$
$$frac29cdotfracpi cdot sin (1/3pi)sin(-2/3pi)cdot sin(-1/3 pi)= frac4pi9sqrt3$$
So:
$$mathrmBleft(frac53,frac43 right) = fracGamma(5/3) Gamma(4/3)Gamma(3) = fracGamma(5/3) Gamma(4/3)(3-1)!=$$
$$= fracGamma(5/3) Gamma(4/3)2=frac2pi9sqrt3$$
In conclusion we obtain:
$$boxeddisplaystyleint limits_a^b sqrt[3](b-x)(x-a)^2 dx = (b-a)^2frac2pi9sqrt3$$
add a comment |Â
up vote
0
down vote
up vote
0
down vote
Thanks @Chappers for an idea. Here is the way of computing the product they was talking about:
We will require two basic properties. The first one is the Euler reflection formula:
$$Gamma(1-z)Gamma(z) = fracpisin z pi$$
The second is $Gamma(z+1)=zGamma(z)$. So,
$$Gammaleft(frac53right) Gammaleft(-frac23 right) = fracpisin left(-frac23 piright)$$
$$Gammaleft(frac43right) Gammaleft(-frac13 right) = fracpisin left(-frac13 piright)$$
We multiply:
$$Gamma(5/3)Gamma(-2/3)Gamma(4/3)Gamma(-1/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)$$
$$Gamma(5/3)Gamma(4/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)Gamma(-2/3)Gamma(-1/3)$$
$$Gammaleft(frac13 right)=Gammaleft(-frac23+1 right)=-frac23Gammaleft(-frac23 right)$$ $$Gammaleft(frac23 right)=Gammaleft(-frac13+1 right)=-frac13Gammaleft(-frac13 right)$$
Hence
$$Gammaleft(-frac13 right) = -3Gammaleft(frac23 right) text ø Gammaleft(-frac23 right) = -frac32Gammaleft(frac13 right)$$
Finally,
$$Gammaleft(frac53right)Gammaleft(frac43right) =displaystylefracpi^2sin left(-frac23 piright) cdot sin left(-frac13 piright)cdot -frac32 Gammaleft(frac13right) cdot(-3)Gammaleft(frac23right)=$$
$$=frac29cdotfracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)underbraceGammaleft(1/3right) Gammaleft(2/3right)_displaystylefracpisin(1/3pi)=$$
$$frac29cdotfracpi cdot sin (1/3pi)sin(-2/3pi)cdot sin(-1/3 pi)= frac4pi9sqrt3$$
So:
$$mathrmBleft(frac53,frac43 right) = fracGamma(5/3) Gamma(4/3)Gamma(3) = fracGamma(5/3) Gamma(4/3)(3-1)!=$$
$$= fracGamma(5/3) Gamma(4/3)2=frac2pi9sqrt3$$
In conclusion we obtain:
$$boxeddisplaystyleint limits_a^b sqrt[3](b-x)(x-a)^2 dx = (b-a)^2frac2pi9sqrt3$$
Thanks @Chappers for an idea. Here is the way of computing the product they was talking about:
We will require two basic properties. The first one is the Euler reflection formula:
$$Gamma(1-z)Gamma(z) = fracpisin z pi$$
The second is $Gamma(z+1)=zGamma(z)$. So,
$$Gammaleft(frac53right) Gammaleft(-frac23 right) = fracpisin left(-frac23 piright)$$
$$Gammaleft(frac43right) Gammaleft(-frac13 right) = fracpisin left(-frac13 piright)$$
We multiply:
$$Gamma(5/3)Gamma(-2/3)Gamma(4/3)Gamma(-1/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)$$
$$Gamma(5/3)Gamma(4/3) = fracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)Gamma(-2/3)Gamma(-1/3)$$
$$Gammaleft(frac13 right)=Gammaleft(-frac23+1 right)=-frac23Gammaleft(-frac23 right)$$ $$Gammaleft(frac23 right)=Gammaleft(-frac13+1 right)=-frac13Gammaleft(-frac13 right)$$
Hence
$$Gammaleft(-frac13 right) = -3Gammaleft(frac23 right) text ø Gammaleft(-frac23 right) = -frac32Gammaleft(frac13 right)$$
Finally,
$$Gammaleft(frac53right)Gammaleft(frac43right) =displaystylefracpi^2sin left(-frac23 piright) cdot sin left(-frac13 piright)cdot -frac32 Gammaleft(frac13right) cdot(-3)Gammaleft(frac23right)=$$
$$=frac29cdotfracpi^2sin (-2/3 pi) cdot sin (-1/3 pi)underbraceGammaleft(1/3right) Gammaleft(2/3right)_displaystylefracpisin(1/3pi)=$$
$$frac29cdotfracpi cdot sin (1/3pi)sin(-2/3pi)cdot sin(-1/3 pi)= frac4pi9sqrt3$$
So:
$$mathrmBleft(frac53,frac43 right) = fracGamma(5/3) Gamma(4/3)Gamma(3) = fracGamma(5/3) Gamma(4/3)(3-1)!=$$
$$= fracGamma(5/3) Gamma(4/3)2=frac2pi9sqrt3$$
In conclusion we obtain:
$$boxeddisplaystyleint limits_a^b sqrt[3](b-x)(x-a)^2 dx = (b-a)^2frac2pi9sqrt3$$
answered Jul 25 at 20:18
Nicholas S
17010
17010
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$$x=acos^2t+bsin^2t$$
– lab bhattacharjee
Jul 25 at 16:41
@labbhattacharjee And then what?
– Mark Viola
Jul 25 at 17:27