Introduction to Plane Geometry
From Latin: planus "flat, level," and Greek: geometrical "measurement of earth or land"

The study of geometry can be broken into two broad types: plane geometry, which deals with only two dimensions, and and solid geometry which allows all three. The world around us is obviously three-dimensional, having width, depth and height, Solid geometry deals with objects in that space such as cubes and spheres.

Plane geometry deals in objects that are flat, such as triangles and lines, that can be drawn on a flat piece of paper.

## The Plane

In plane geometry, all the shapes exist in a flat plane. A plane can be thought of an a flat sheet with no thickness, and which goes on for ever in both directions. It is absolutely flat and infinitely large, which makes it hard to draw. In the figure below, the yellow area is meant to represent a plane. In the figure, it has edges, but actually, a plane goes on for ever in both directions.

Try this Drag the rectangles around on the yellow plane P. Move the blue rectangle, notice it does not lie in the yellow plane P
(If there is no image below, see support page.)

The rectangles ABCD and PQRS are in the plane p. You can drag them around in the plane with the mouse. Because they both exist in the same plane, they are said to be coplanar.

The blue rectangle however is not in the plane p. If you move it with the mouse you can see it exists in a different plane (not shown) that is vertical with respect to p. The blue rectangle is not coplanar with ABCD or PQRS.

## Origins

Plane geometry, and much of solid geometry also, was first laid out by the Greeks some 2000 years ago. Euclid in particular made great contributions to the field with his book "Elements" which was the first deep, methodical treatise on the subject. In particular, he built a layer-by-layer sequence of logical steps, proving beyond doubt that each step followed logically from those before.

Geometry is really about two things:

1. The objects and their properties. Analysis of things such as points, lines, triangles.
2. Proofs. A methodology for proving that the claims made about objects are really true.

Clearly, our world is three dimensional. But in the fictional story Flatland by Edwin Abbott, he speculates what living in a two-dimensional world (a plane) would be like. It's a fun diversion from the strict factual logic of mathematics. Surprisingly for a science fiction story, it was written in 1884, and his writing style is quaintly Victorian as a result. An excerpt from Chapter 1:

..Imagine a vast sheet of paper on which straight Lines, Triangles, Squares, Pentagons, Hexagons, and other figures, instead of remaining fixed in their places, move freely about, on or in the surface, but without the power of rising above or sinking below it, very much like shadows ...

Read it online at "Flatland - A romance of many dimensions" by Edwin A. Abbott, a Square,1884;

## Other line topics

COMMON CORE

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See which resources are available on this site for each element of the Common Core standards.

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