Introduction to Coordinate Geometry
A system of geometry where the position of
points
on the
plane
is described using an ordered pair of numbers.
Recall that a plane is a flat surface that goes on forever in both directions. If we were to place a point on the plane,
coordinate geometry gives us a way to describe exactly where it is by using two numbers.
What are coordinates?
To introduce the idea, consider the grid on the right. The columns of the grid are lettered A,B,C etc.
The rows are numbered 1,2,3 etc from the top. We can see that the X is in box D3; that is, column D, row 3.
D and 3 are called the coordinates of the box. It has two parts: the row and the column.
There are many boxes in each row and many boxes in each column. But by having both we can find one single box,
where the row and column intersect.
The Coordinate Plane
In coordinate geometry, points are placed on the "coordinate plane" as shown below.
It has two scales  one running across the plane called
the "x axis" and another a right angles to it called the y axis. (These
can be thought of as similar to the column and row in the paragraph above.)
The point where the axes cross is called the origin and is where both x and y are zero.
On the xaxis, values to the right are positive and those to the left are negative.
On the yaxis, values above the origin are positive and those below are negative.
A point's location on the plane is given by two numbers,the first tells where it is on the xaxis and the second
which tells where it is on the yaxis. Together, they define a single, unique position on the plane. So in the diagram above,
the point A has an x value of 20 and a y value of 15. These are the coordinates of the point A, sometimes referred to as its
"rectangular coordinates". Note that the order is important; the x coordinate is always the first one of the pair.
For a more indepth explanation of the coordinate plane see The Coordinate Plane.
For more on the coordinates of a point see Coordinates of a Point
Things you can do in Coordinate Geometry
If you know the coordinates of a group of points you can:
 Determine the distance between them
 Find the midpoint, slope and equation of a line segment
 Determine if lines are parallel or perpendicular
 Find the area and perimeter of a polygon defined by the points
 Transform a shape by moving, rotating and reflecting it.
 Define the equations of curves, circles and ellipses.
Information on all these and more can be found in the pages listed below.
History
The method of describing the location of points in this way was proposed by the French mathematician René Descartes (1596  1650).
(Pronounced "day CART"). He proposed further that curves and lines could be described by equations using this technique,
thus being the first to link algebra and geometry. In honor of his work, the coordinates of a point are often referred to as its
Cartesian coordinates, and the coordinate plane as the Cartesian Coordinate Plane.
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