I've never had problems with math in my life. I absolutely love it and never got bored of studying it in high school. During my forth year in high school, I had a very good math professor and asked him if "elements of set theory and functional analysis" by kolmogorov was a good book to study from. He said yes and I started studying it. At one point, at the beginning of the second chapter, the author writes this identity and says that it's "easy to demonstrate", so I demonstrated it. I only used one pen, one piece of paper, no calculator and no internet. I found it not so easy but not overly difficult. Then I stopped studying the book because I had to focus on school subjects. During the fifth year, this great professor was no lo got out teacher and another(a very terrible one) came to teach us. Basically, a whole year with no serious math at all. Now, one year later, I have finished high school and took this book back to study it, since I'm starting physics university at September. I arrived at this same identity and I can no lo get demonstrate it. It thought that I simply got rusty, so I took the old demonstration I made to check it. Apparently, I can no longer understand the demonstration that I did. Is it normal to get THIS rusty?
