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# Integrating fraction without ln Watch

1. By using substitution which is recommended in the question, I have tried u=1+x for

(1-x)/(1+x)^4

But I end up with a log, which isn't in the answer. Am I using the wrong substitution?
2. Nope, substitution is fine. Write out your solution so we can see where you're going wrong
3. (Original post by SimonM)
Nope, substitution is fine. Write out your solution so we can see where you're going wrong
u=1+x
x=u-1

du=dx

then integrate 1/(u^4)du = 4lnu
multiply by 1-(u-1) which is u (as this is the 1-x at the top)

so (1+x)4ln(1+x)

4. Firstly the integral of u^(-4) isn't 4 ln u

Secondly, that's not the integral you should be finding!

(1-x)/(1+x)^4 = (2-u)/u^4
5. (Original post by SimonM)
Firstly the integral of u^(-4) isn't 4 ln u

Secondly, that's not the integral you should be finding!

(1-x)/(1+x)^4 = (2-u)/u^4
Ah I see now, thank you

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Updated: December 19, 2010
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