2) Basic Elements of Geometry
Points

Perhaps the most basic geometric shape is the point. A point is represented by a dot. You can draw a point on a piece of paper, and in geometry we usually give point single-letter names, using capital letters. In algebra you would study points on a graph, and besides having a capital letter for a name, the point would have two numbers associated with it, representing the horizontal distance and the vertical distant of the point from the"origin", as you can see in the graph below. The first number is called the "x-coordinate" and the second number is called the "y-coordinate.Therefore, the point A is 2 units to the right of the origin, and 3 units up.

If you look, you can find many examples of graphs in the newspaper and on the internet, showing scientific, financial and mathematical data.

Lines

We can draw a straight line through two points. Lines are usually named using the names of the two points that determine the line. We would then call the geometric figure "line AB" or "line BA" as shown in the example below, of a line formed by the window frames of a high-rise office building.

If two lines intersect, four angles are formed as in the diagram below. An angle has two sides (lines) and a vertex (a point). We name an angle using the letter for a point on one side, the vertex, then a point on the other side, as shown below. So the four angles in this diagram would be called "angle FDE (or angle EDF), angle CDF, angle EDG, and angle CDG".

As you look around, you can find many examples of angles, such as the angle (labeled angle ABC) formed by two of the petals of this Bird of Paradise flower.


Go to Topic 3 Polygons