- #1
Samuelb88
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Homework Statement
Solve the 1st order linear initial value problem.
[tex](1+x^2)\frac{dy}{dx}\right) + 4xy = x [/tex] , [tex]y(1)=\frac{1}{4}\right)[/tex]
The Attempt at a Solution
Why can I get the answer solving the equation one way (below), but not they other?
Method 1:
[tex](1+x^2)\frac{dy}{dx}\right) = -x(4y-1)[/tex]
[tex]\frac{dy}{dx}\right) * \frac{1}{4y-1}\right) = -\frac{x}{1+x^2}\right)[/tex]
[tex]\frac{1}{4}\right) \int^{}_{} \frac{1}{4y-1}\right) d(4y-1) = -\frac{1}{2}\right) \int^{}_{} \frac{1}{1+x^2}\right d(1+x^2)[/tex]
(multiply by 4)
[tex]ln|4y-1| = -2ln|1+x^2| + C[/tex]
[tex]ln|(4y-1)(1+x^2)^2| = 4C[/tex]
[tex] 4y-1 = \frac{e^4^C}{(1+x^2)^2}\right)[/tex]
Satisfying y(0)=1/4, C=0
Therefore: y = 1/4Method 2:
[tex](1+x^2)\frac{dy}{dx}\right) = -x(4y-1)[/tex]
[tex]\frac{dy}{dx}\right) * \frac{1}{4y-1}\right) = -\frac{x}{1+x^2}\right)[/tex]
[tex]\frac{1}{4}\right) \int^{y(x)}_{1/4} \frac{1}{4y-1}\right) d(4y-1) = -\frac{1}{2}\right) \int^{x}_{1} \frac{1}{1+x^2}\right d(1+x^2)[/tex]
[tex]( ln|4y-1| - ln|0| ) = -2(ln|1+x^2| - ln|2|)[/tex]
Here's where I'm confused. should I ignore ln0? or should I solve with infinity in my equation.