Perimeter

The perimeter of a square or a triangle is the distance all the way around the edge. It's the same as the circumference of a circle. The perimeter of a quadrilateral, like a square or a rectangle, is always equal to twice the length plus twice the width. The perimeter of a triangle is always equal to adding the length of all three sides together.
Squares
Triangles
Circles
Area
Main Geometry page
To find out more about geometry, check out these books from Amazon.com or from your library:
