Background Essay: Solving for X and Y

The coordinate plane is a flat surface that is divided by two lines into four quadrants. The horizontal line is called the x-axis, and the vertical line is called the y-axis. These two axis lines intersect at right (90^o) angles.

The x-axis and the y-axis are marked as number lines with both positive and negative numbers, so the coordinate plane looks like a grid. The positive numbers lie to the right on the x-axis and upward on the y-axis. The point where the lines intersect is called the origin. The origin is located at zero on both number lines.

Every point on the grid has an address, called an ordered pair, which gives the coordinates for a point on the plane. An ordered pair is always given in the order (x,y). The x-coordinate defines the position of the point on the x-axis. The y-coordinate defines the position of the point on the y-axis.

Because there is a relationship between the variables x and y, you can use their coordinates to solve equations with two variables. Solutions to equations with two variables can be written as ordered pairs (x,y) and represented in a table. Ordered pairs in a table can be plotted as points and connected with a line on a graph.