Integrating Factor for First Order ODE/Conclusion

Theorem
Let the first order ordinary differential equation:
 * $(1): \quad \map M {x, y} + \map N {x, y} \dfrac {\d y} {\d x} = 0$

be non-homogeneous and not exact.

Let $\map \mu {x, y}$be an integrating factor for $(1)$.

If one of these is the case:
 * $\mu$ is a function of $x$ only
 * $\mu$ is a function of $y$ only
 * $\mu$ is a function of $x + y$
 * $\mu$ is a function of $x y$

then:
 * $\mu = e^{\int \map f w \rd w}$

where $w$ depends on the nature of $\mu$.

Proof
We have one of these:
 * Integrating Factor for First Order ODE: Function of One Variable: $x$ or $y$ only
 * Integrating Factor for First Order ODE: Function of $x + y$
 * Integrating Factor for First Order ODE: Function of $x y$

We have an equation of the form:
 * $\dfrac 1 \mu \dfrac {\d \mu} {\d w} = \map f w$

which is what you get when you apply the Chain Rule for Derivatives and Derivative of Logarithm Function to:
 * $\dfrac {\map \d {\ln \mu} } {\d w} = \map f w$

Thus:
 * $\ds \ln \mu = \int \map f w \rd w$

and so:
 * $\mu = e^{\int \map f w \rd w}$

Hence the results as stated.