My math skills were always bad. I mean, even though I had one of the best scores in school, I could never explain why it works, e.g. why <i>9^1/2</i> is the same as <i>sqr(9)</i>.<p>Since I want to self-study the <i>Quant Finance</i>, I decided to refresh and develop my math skills up to the Calculus. This website is amazing! Not only it's a good refresher, it's the best source of knowledge created by one man.