First things first. I will allow reproduction and distribution of the blog's contents for any non-profit or personal use. This blog is a storage of the essential things that I have learned which I don't want to lose and also for helping students - upgrading or high school - improve their grades. Everything here is for free. The knowledge is free. Techniques are free. If anyone intends to sell or copy any of its contents or its structural organization for profit or selfish purposes that will take advantage of other individuals, well, shame on you. Let's get started!