Coordinates
Coordinates are an ordered set of numbers that define the position of a point. If the point is on a plane, then two numbers are used. To define the position of a point in threedimensional space, we need three.
On a plane (2D)
To define the position of a point on a 2D plane, we use two numbers, called the xcoordinate and the ycoordinate.
The xcoordinate tells where the point is in the leftright direction, and the ycoordinate tells where it is in the vertical, updown direction.
In the figure above the coordinates of the point A are 20, 15. The first (x) coordinate tells how far along the horizontal (x) axis it is, here 20. The second (y) coordinate tells how far up the vertical (y) axis it is, here 15. The x coordinate is always first in the pair.
For more on this see Coordinates of a point on the plane.
In space (3D)
To define a point in 3D space we add a third coordinate called z. This tells how far the point is in the third dimension.
You can think of this third (z) dimension as going in and out of the page.
A method for finding the area of any polygon  regular, irregular, convex, concave if you know the coordinates of the vertices.
Description of how the position of a point can be defined by x and y coordinates.
This calculator takes the parameters of a regular polygon and calculates its coordinates. It produces both the coordinates of the vertices and the coordinates of the line segments making up the sides of the polygon.
Finding the distance between two points given their coordinates
Definiton of the equation of a straight line, in 'point  slope' form: y = m(xPx) + Py
Definiton of the equation of a straight line, in 'slope and intercept' form: y = mx+b
How to find the intercept of a line, given the coordinates of two points on the line
Introduction to coordinate geometry.
Introduction to the concept of lines in coordinate geometry.
Definition of a line when the defining points are on the coordinate plane
Definition of a line segment when the defining points are on the coordinate plane
Finding the midpoint of a line segment given the coordinates of the endpoints
Definition of 'origin' and its relationship to coordinate geometry
Print blank graph paper  Cartesian (rectangular) coordinates
Definition of a ray when the defining points are on the coordinate plane
Definition of the slope of a line given the coordinates of two points on the line, includes slope as a ratio and an angle.
Limitations of the applets in the coordinate geometry section
The coordinate plane defined with description of x,y axis, quadrants, origin.
(C) 2011 Copyright Math Open Reference. All rights reserved
