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 Math Emeritus Sci Advisor Thanks PF Gold P: 39,345 Integrating Factor, how do you get this? As I said, they didn't "drop" mu, they solved for it! If $$\frac{du}{dx}= F(x)u$$ then $$\frac{du}{u}= F(x)dx$$ and, integrating both sides, $$ln(u)= \int F(x)dx$$ Now take the exponential of both sides.