First order differential equation
Maths and statistics discussion, revision, exam and homework help.
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First order differential equationI need to solve:
Ln(y^x). dy/dx = 3x^2 . y
Rearrange to get
X.lny (dy) = 3x^2 (dx)
Int (X.lny (dy)) = Int (3x^2 (dx))
To get
(lny)^2/2 = 3x^2/2 + C
According to my mark scheme I should get C/2
But I don't know why? -
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Re: First order differential equation
- try integrating by parts (where u = ln y, and dv/dy = 1)
Also, I don't see where your y from the first line has gone.
Edit: And ignore the first two parts as they have been corrected below - next time try and keep the y in your working throughout, but equally (if not more so) it's my bad for not trying with it first
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Last edited by starkrush; 19-04-2012 at 16:57. -
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Re: First order differential equationTo integrate ln(y)/y use substitution:(Original post by claret_n_blue)
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let u= lny
du/dy = 1/y
du= dy/y
-> I u du = u^2/2= (lny)^2/2. -
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Re: First order differential equationhttp://www.wolframalpha.com/input/?i...dx+%3D+3x%5E2y
Wolfram says no constant/2.
Not quite sure where that constant/2 has come from, I certainly don't get that either. -
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Re: First order differential equationWrong.(Original post by starkrush)
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OP is correct, if I'm interpreting correctly.Last edited by f1mad; 19-04-2012 at 16:46. -
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Re: First order differential equationLast edited by f1mad; 13 Minutes Ago at 16:46(Original post by starkrush)
Ninja edited 1 minute before you replied
And edited to reflect this clearer
Last edited by starkrush; 1 Minute Ago at 16:57.
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