# Homework Help: Playing With Logs while solving y-x=5(ln|y+3|-ln|x+4|) for y.

1. Sep 25, 2013

### gikiian

1. The problem statement, all variables and given/known data

The problem is actually the following Separable Differential Equation:
$\frac{dy}{dx}=\frac{xy+3x-y-3}{xy-2x+4y-8}$​
I am required to find y(x).

2. Relevant equations and techniques

- factorization (applied on the numerator and the denominator in the problem equation)
- basic integration (applied after separating the variables)
- at least the following logarithmic identities:
$e^{ln|x|}=x$ ; $ln|xy|=ln|x|+ln|y|$​

3. The attempt at a solution

$\frac{dy}{dx}=\frac{xy+3x-y-3}{xy-2x+4y-8}$

$(1-\frac{5}{y+3})dy=(1+\frac{5}{x+4})dy$

$y-5ln|y+3|=x+5ln|x+4|+C$

$y-x=5(ln|y+3|-ln|x+4|)$

I also put this result in Wolfram Alpha, but it could not solve the equation for y. My instructor apparently believes that an y=f(x) can be obtained. Is there any way to do so?

Last edited: Sep 25, 2013
2. Sep 25, 2013

### haruspex

You appear to have a sign wrong in the middle two equations, but since it comes out right at the end maybe this was a copying out error.
i don't see a way to get it into y = f(x) form either.