I was doing this problem in calculus, and I came up with the answer dy/dx=n(n+1)(n+2)..(n+(n-1)) and so on. To me this looked like a factorial backwards, and I wanted to find notation that expresses such a series. I googled it and I found the "Rising Factorial" notation. However, as I continued reading the page, I saw this, "The rising factorial can be expressed as Factorial of (n+(n-1))." At this moment I felt stupid for not realising this. It was obvious yet I didn't see it. Now why I am posting this topic is that I get really frustrated when such things occur, when I just don't see it. It doesn't happen frequently, but when it does it just kills me and makes me think I am stupid or something. I even considered choosing another major due to this happening to me before. I know I may be overreacting, and I know I am not stupid, I am well capable of mathematics and physics beyond my age, but I don't know how to deal with such moments. Does it ever happen to you? how do you deal with it then?