This is another solution method that is both a more general form of the separation of variables, and a more powerful form that extends your abilities to address other types of problems.
While the derivation is detailed, and difficult to take it all in, the resulting method is clean and aggressive. Little by little, as we progress through these things, you will see more and more of why we went through the derivation - because each step in it contains its own set of specific solutions. I know it was rough. Think back to the Fundamental Theorem discussions. They were rough too. But it is all simple now and part of your instincts and mathematical vocabulary. This stuff will progress that way also.
Download Exp Int Factor Intro 3-11