Recently, I decided to relearn elementary differential equations. I began to read a differential equations book I have, published in 1929 (it was my Father's book).
One type of solution method involves the use of the differential operator, D, in symbolic form. It's nifty, and once I had some of the basics, I decided to do one of the book's problems that way.
The particular problem looked pretty simple, and the book gave the answer, which I was supposed to verify.
I had a nice elegant and simple approach to solving the problem, and proceeded to apply it. I did not get the book's answer.
Over the next few days, I redid my solution and reviewed it, endlessly. I am error prone, and did find careless errors on my part. Nevertheless, I did not get the book's answer, although I did, the last few times, get the same "my" answer.
I decided to double check everything one final time. Glancing at the book, and at my worksheet, I noticed that I had copied the problem wrong. Yeesh!!!
I used my approach to solve the right problem in my head in five minutes.
Such is life when you are 77 years old.