National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Constructing the Orthocenter
Dr. Math shows how to construct the orthocenter of any triangle using a compass and straight edge.
Cut the Knot
Cut the Knot!: Proofs in Mathematics
This site provides an explanation of mathematical proofs. It also has links to different types of proofs.
Tech Target
Whatis.com: Cartesian Coordinates
This site from Whatis.com provides a great definition of cartesian coordinates. The definition is medium-sized in length and has links to additional information as well.
University of Illinois
University of Illinois Urbana Champaign: Torus Math: Spherical Geometry Demo
A java applet on this site illustrates the principles or spherical geometry.
Michigan State University
Michigan State University: Biometrics: Hand Geometry
Students use hand geometry to prove their own identification.
Wolfram Research
Wolfram Math World: Hyperbolic Geometry
Hyperbolic Geometry is discussed here with specifics on the lines in this geometry. The equations of the lines are represented as well as a desription of the lines. The site has links to other non-Euclidean Geometries.
Wolfram Research
Wolfram Math World: Two Column Proof
MathWorld gives a discussion of the use of two-column proofs by high school and middle school students and its relevance to mathematicians.
Wolfram Research
Wolfram Math World: Circumcenter
MathWorld diagrams a triangle with its circumcenter labeled and details how to find the circumcenter with links to the key terms.
Math Words
Math words.com: Elliptic Geometry
The basic definition for what elliptic geometry is is found here.