A helpful scientific calculator that runs in your web browser window.
Holt Online Learning

Trinagles can be classified according to sides and angles
Math Open Reference

The conventional method of calculating the area of a triangle (half base times altitude) with pointers to other methods and special formula for equilateral triangles
Math Open Reference

A page with links for properties of circles and different types of polygons. Each link has great visuals with clear markings, definitions, and formulas.
Cool Math

Worksheet to practice classifying triangles as scalene, isosceles, or equilateral.
Helping with Math

The worksheets below can be used as part of extra math homework. These geometry worksheets are free and easily printable. They cover typical school work from grade 4 through 8. They include questions on polygons, 3D objects, angles, and calculations of area, volume, etc.
Helping With Math.com

Formulas about sin, cos, tan, csc, sec, and cot
Math.com

An introduction the the concept of solving a triangle.
Math Open Reference

Formula and description of the perimeter of a triangle.
Math Open Reference

Definition and properties of the midsegment of a triangle
Math Open Reference
