FP3 DEs by substitution questionWatch

Thread starter 3 years ago
#1
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Can anyone help me with this question? I can get as far as du/dx = u^3/(3x)-4u/(3x) but I'm not sure if it's correct up to here and if it is I'm struggling to integrate the separated variables.

Part D:
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3 years ago
#2
(Original post by Rabadon)
Can anyone help me with this question? I can get as far as du/dx = u^3/(3x)-4u/(3x) but I'm not sure if it's correct up to here and if it is I'm struggling to integrate the separated variables.

Part D:
Could you post your working?
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Thread starter 3 years ago
#3
(Original post by Muttley79)
Could you post your working?
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3 years ago
#4
(Original post by Rabadon)
Just checking the end - can you collect the ln with the RHS?

You are making hard work of the partial fractions - why did you expand?

1 = A (u + 2)(u -2) + Bu (u - 2) + Cu (u + 2)

then subst in u = 0 then other sensible values - much quicker.
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Thread starter 3 years ago
#5
(Original post by Muttley79)
Just checking the end - can you collect the ln with the RHS?

You are making hard work of the partial fractions - why did you expand?

1 = A (u + 2)(u -2) + Bu (u - 2) + Cu (u + 2)

then subst in u = 0 then other sensible values - much quicker.
Yeah we've hardly covered partial fractions so my bad . Anyway, they got the same as me in the back apart from a few questions I have.

I've integrated 1/3x incorrectly, but it's kind of confusing me. 1/3 could be taken out of the integrand and then it integrates to 1/3ln(X), however if the 3 is left in the integrand it integrates to 1/3ln(3x). Does the fraction have to be in its simplest terms to integrate like this idk.

Also, the attached answers aren't in the form y=f(X) which I thought was necessary. At what point is it ok to stop and not put it in that form?
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3 years ago
#6
(Original post by Rabadon)
Anyway, they got the same as me in the back apart from a few questions I have.

I've integrated 1/3x incorrectly, but it's kind of confusing me. 1/3 could be taken out of the integrand and then it integrates to 1/3ln(X), however if the 3 is left in the integrand it integrates to 1/3ln(3x). Does the fraction have to be in its simplest terms to integrate like this idk.

Also, the attached answers aren't in the form y=f(X) which I thought was necessary. At what point is it ok to stop and not put it in that form?
What happens if you differentiate 1/3 ln(3x) .... you don't get 1/3x, so its best to take out the 1/3.

Once you subsitute back it usually easy to see if the answer can be given as y = f(x) and here it can't. So just tidy up as they have done.
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Thread starter 3 years ago
#7
(Original post by Muttley79)
What happens if you differentiate 1/3 ln(3x) .... you don't get 1/3x, so its best to take out the 1/3.

Once you subsitute back it usually easy to see if the answer can be given as y = f(x) and here it can't. So just tidy up as they have done.
ok awesome thank you!

Can I be cheeky and ask you what I've done wrong here too?? (what looks like 1/(8p) is meant to be (1/8)p btw

edit: oh I need to take the 13/8 out before I use f'(X) rule
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3 years ago
#8
(Original post by Rabadon)
ok awesome thank you!

Can I be cheeky and ask you what I've done wrong here too?? (what looks like 1/(8p) is meant to be (1/8)p btw
Of course!

Again, it's a slip I think - if you integrate 2dp you get 2p + c so if you integrate (1/8)dp you get (1/8)p not 1/8p.

I always suggest differentiating if you aren't sure or want to check.
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Thread starter 3 years ago
#9
(Original post by Muttley79)
Of course!

Again, it's a slip I think - if you integrate 2dp you get 2p + c so if you integrate (1/8)dp you get (1/8)p not 1/8p.

I always suggest differentiating if you aren't sure or want to check.
ah yes, I realised that pretty quickly. I also made another error integrating the other thing. I need to remember to take things out of the integrand before using that f(x) f'(x) rule :/
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