DE is for “Desktop environment”. Most distro let you choose from quite a few. The two best known one are Gnome and KDE, i believe. But there are many more.

“Desktop environment” might just be two words without much meaning to you. If that the case, in the linux context, a desktop environment is basically the GUI to your computer: file explorer, desktop, taskbar, general setting manager and so on.

I’m just being a pendant here, but the derivative of x²+C is 2x. You put the constant at the wrong place.

Also, i’m glad you found a textbook well suited for you. I have to wonder what you mean by ‘why’, do you mean a proof?