These old notes served me well. Throughout engineering school, there was a constant theme of learning new mathematical techniques and then leaving them behind just long enough to forget them.
…it was precisely this trend that drove me to creste my own math book. So many of the techniques I learned required some unique or special explanation in order for me to understand. When a topic would come back up, relesrning could be a painfully slow process if I couldn’t find the specific explanation that “clicked” for me - so I made a book to hold all the explanations that made sense to me, so I could always quickly look back and remember the process.
As an engineer today, it is with some (negligible) sadness that I no longer reference this book. Not because I’ve become so good at mathin’, but rather, because I really haven’t used these techniques over my last 10 years in the field!