integration of irrational function


by KLscilevothma
Tags: function, integration, irrational
KLscilevothma
KLscilevothma is offline
#1
Jun7-03, 11:39 PM
P: 321
Here's the question that I got stuck:

[inte]sqrt[x/(a-x)] dx ......................................(*)

I tried to use the following substitution
u=sqrt[x/(a-x)] and .........................................(1)
dx = 2u(1-a)/(1+u2)2 du................(2)

sub (1) and (2) into (*), after a few steps, I got

(2-2a)[inte]du/(1+u2) - 2(1-a)[inte]du/(u2+1)2

The answer derived from the first part, (2-2a)[inte]du/(1+u2), contains tan -1 but the model answer of this question is
-[squ](ax-x2) + a/2sin-1[(2x+a)/a] + C
For the second part, I let u = tan θ and got a strange expression.

Is my approach correct and is the final answer obtained from the above method differs the model answer only by the constant of integration ? Or am I using a wrong substitution?
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Hurkyl
Hurkyl is offline
#2
Jun8-03, 09:17 AM
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Check your work on your substitution.
KLscilevothma
KLscilevothma is offline
#3
Jun9-03, 07:00 AM
P: 321
u=sqrt[x/(a-x)]

dx = 2au/(1+u2)2 du

thanks


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