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Using the substitution x=(1-u)^2, find *integral*--> 1/(1-√x) dx

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Original post by calster102
Using the substitution x=(1-u)^2, find *integral*--> 1/(1-√x) dx


Show your working.
Reply 2
Can you lead me on the right path ie. start me off because then i'll easily do the rest :frown: this is such a stubborn one i've done all the other questions.
Original post by calster102
Can you lead me on the right path ie. start me off because then i'll easily do the rest :frown: this is such a stubborn one i've done all the other questions.


You start the same way as any sub question

sub u in

find dx in terms of u and du and sub that in
Reply 4
you're saying to sub u in but we've been given x as a substitution :frown: :frown:
Reply 5
Original post by calster102
you're saying to sub u in but we've been given x as a substitution :frown: :frown:


Same difference in effect, but in this case it's fairly straightforward:

substitute x = ... into the function you're integrating and see what it turns into as a function of u
Work out dx/du and use this to replace the dx by (something) x du
Reply 6
Original post by davros
Same difference in effect, but in this case it's fairly straightforward:

substitute x = ... into the function you're integrating and see what it turns into as a function of u
Work out dx/du and use this to replace the dx by (something) x du


I subbed x in and I got 1/1-√(1-u)^2 and for dx/du is it u^2-2u?
Original post by calster102
I subbed x in and I got 1/1-√(1-u)^2 and for dx/du is it u^2-2u?


ok - if it is easier for you to think that way

So - That is not correct for dx/du I am not really sure how you got that
Reply 8
Original post by TenOfThem
ok - if it is easier for you to think that way

So - That is not correct for dx/du I am not really sure how you got that


Please enlighten me on your methods :smile:
Original post by calster102
I subbed x in and I got 1/1-√(1-u)^2 and for dx/du is it u^2-2u?


dxdu=2(1u)\frac{dx}{du} = -2(1-u)

Can you go from there?
Reply 10
Original post by SamKeene
dxdu=2(1u)\frac{dx}{du} = 2(1-u)

Can you go from there?


Ohh I see I expanded it out silly me so you'd replace dx by this in the integral
Original post by calster102
Ohh I see I expanded it out silly me so you'd replace dx by this in the integral


yes
Original post by calster102
Ohh I see I expanded it out silly me so you'd replace dx by this in the integral


Rearrange so its dx = {} but yes, also note I missed out a negative in my working, I've corrected it in the post.
Original post by calster102
Please enlighten me on your methods :smile:


there is no different method

You subbed in a function of u

You did not sub in x

it is just terminology - you were doing the right thing
Reply 14
right i seem to have got -2ln(1-√x)+2(1-√x)+C ?? :frown:
Original post by calster102
right i seem to have got -2ln(1-√x)+2(1-√x)+C ?? :frown:


yep
Reply 16
On symbolab which is this partially decent site they use u=√x and end up with -2ln(√x-1)-2√x +C? so can you get different indefinite integrals depending on which substitution you take???
Reply 17
Original post by calster102
On symbolab which is this partially decent site they use u=√x and end up with -2ln(√x-1)-2√x +C? so can you get different indefinite integrals depending on which substitution you take???


Yes provided the difference is only a constant, which in this case it is

(There is a slight subtlety here in that one of the logs only works if root(x) - 1 > 0 and the other only works if 1 - root(x) > 0 but you're not expected to worry about this in this particular question :smile: )
Reply 18
Original post by davros
Yes provided the difference is only a constant, which in this case it is

(There is a slight subtlety here in that one of the logs only works if root(x) - 1 > 0 and the other only works if 1 - root(x) > 0 but you're not expected to worry about this in this particular question :smile: )


Thanks did you end up with the same answer as me right?
Original post by calster102
Thanks did you end up with the same answer as me right?


Using the sub they gave your answer is fine


Clearly if you have 2+c you can just write +c as this is constant


Also in both yours and theirs it should have said ln|blah| and the modulus of 1x1-\sqrt x is the same as the modulus of x1\sqrt x - 1
(edited 9 years ago)

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