Definition of Area and Perimeter http://www.khanacademy.org/math/geometry/basic-geometry/perimeter_area_tutorial/v/perimeter-and-area-basics Finding Perimeter in a Coordinate Plane...
Electrical Engineering Community's Tools - geometry reference sheet - many more on this site
Algebra Tool Kit Reference Sheet - Free Printable Cheat Sheets. Four pages of easy-to-memorize algebra formulas.-Homeschool Encouragement
Algebra formula chart.
Handy Chart with Math Formulas.
Mensuration RS Aggarwal Class 7 Maths Solutions Exercise 20C http://www.aplustopper.com/mensuration-rs-aggarwal-class-7-maths-solutions-exercise-20c/
A good breakdown of a few of the most notable relativistic physics equations. They really are rather simple, which is why they are so famous and appreciated.
Circle Formulas that are frequently used. The symbolic notation is briefly explained, and the parts are color coordinated. I'd love to print this off and hang it while teaching geometry!
Geometry Formulas Cheat Sheet | eocgeom05geomformulas.gif