Because when I was learning this course, there are too much issues about mathematic problem, but I can't get answer in course media, because my problem seems too naive to ask, so I try to get help from my friends who are very familiar with mathematic expression, and I try to get every things could be understood by very junior developer including very simple derivation. Hope that will help other developers to understand the algorithm which seems like very difficult to understand but actually is just because of it's naming and mysterious math expression.