An integral to calculate

Post Reply
YinZhe Ma
Posts: 11
Joined: October 09 2008
Affiliation: University of KwaZulu-Natal

An integral to calculate

Post by YinZhe Ma » December 23 2015

I have a simple and native equation, but really struggle to find the way to it. Can anyone tell me how to calculate this integral
[tex]
\int^{x}_{0}\frac{y^{3/2} d y}{(1+y)^{3/2}}
[/tex]
I have trying various different ways to solve it, including changing variables from x to sinh function, cosh function etc., but failed. However, I tried it on mathematica and it indeed can find a solution:
[tex]
\frac{\sqrt{x}(x+3)}{(1+x)^{1/2}}-3 \ln ( \sqrt{1+x}+\sqrt{x} )
[/tex]

Please let me know if you have a smart way of doing the integral. this integral is crucial to solve the growth factor in the open universe model.

Eric Linder
Posts: 49
Joined: August 02 2006
Affiliation: UC Berkeley

An integral to calculate

Post by Eric Linder » December 26 2015

You were on the right track. Use the substitution [tex]y=\sinh^2 x[/tex] and it all falls out. The integral becomes
[tex]\int dx [2 \cosh^2 x - 4 + (2/\cosh^2 x)]=(1/2)\sinh(2x)+x-4x+2\sinh x/\cosh x[/tex].
Since [tex]\sinh(2x)=2\sinh x\,\cosh x[/tex] and [tex]x=\sinh^{-1}(\sqrt{y})[/tex] then you get the Mathematica result.

YinZhe Ma
Posts: 11
Joined: October 09 2008
Affiliation: University of KwaZulu-Natal

An integral to calculate

Post by YinZhe Ma » December 27 2015

Thank you Prof.Linder, I get it.

Post Reply