User defined Functions with CAS

[mabajas76]

Hi! I thought that I would start this thread for anyone who was looking/ had some interesting user defined functions for there math subjects.
For those who are not aware, a CAS calculator is an insane tool that when used correctly can provide some life changing help. One of those features are user defined functions, or UDF. Basically, you can create a short cut that will get your calculator to solve specific problems instantly. For example, you are given three points and need to create a quadratic expression? There is a function which will cut out all the time and all you need to do is put the points in and viola! But of course, you need to actually define the function before hand (takes like 2 seconds if you know what it is) and also save and do the working out on the same document (can be different pages)
If anyone has any or wants to know how to do any, post them here!