Wolfram Research
Wolfram Math World: Diagonal Matrix
This site gives a definition and examples of a diagonal matrix. The site also contains many links to other related Math World sites.
Texas Instruments
Texas Instruments: Numb3 Rs: Regular Polygon Centroids
Based off of the hit television show NUMB3RS, this lesson shows students different ways to find centroids of regular polygons. The traditional way (using medians) is presented, as well as the triangulation method, the diagonals method,...
Geometry Help
Geometry Help: Squares
Explains the basic concept of a square with step-by-step instructions and methods for solving specific problems on your own relating to this topic.
University of Cambridge
University of Cambridge: Nrich: Diagonal Trace
Develop problem solving skills working with pentagons and then check your results.