AutoCAD Useful Commands

Tracking Point ---> "tk"
It helps you to locate a  desirable Point in space. Starting from an End Point
without any sketch lines you can dynamic insert any value you want as you pull over your cursor in any direction you desire. 

Properties Palettes --->  "Ctrl+1"
It's display the properties changes of a selected geometry and it lets you to modify your Layer, Points, Dimension, Scale, etc.

Enter Snap
Specify snap spacing or [On/Off/Aspect/Rotate/Style/Type] <5>: s (Style)
Enter snap grid style [Standard/Isometric] <S>: i (Isometric)
There are appears now three isometric angles that you can draw.
Set them through the F5 or "Ctrl+E".


 _ellipse (Iso Circle )
Specify axis endpoint of ellipse or [Arc/Center/Isocircle]: enter i (Isocircle) right-click
Specify center of isocircle: pick or enter coordinates
Specify radius of isocircle or [Diameter]: enter a number

Recall 
This command gives you the ability (when select an object) to call the same command that have been used for the selected object.

Wolfram Demonstrations Project

 An alternative view in science

  Conceived by Mathematica creator and scientist Stephen Wolfram as a way to bring computational exploration to the widest possible audience, the Wolfram Demonstrations Project is an open-code resource that uses dynamic computation to illuminate concepts in science, technology, mathematics, art, finance, and a remarkable range of other fields.