Key Concepts

• Ordered Pair  An ordered pair, [latex]\left(x,y\right)[/latex] gives the coordinates of a point in a rectangular coordinate system.[latex]\begin{array}{c}\text{The first number is the }x\text{-coordinate}.\hfill \\ \text{The second number is the }y\text{-coordinate}.\hfill \end{array}[/latex]
• Steps for Plotting an Ordered Pair (x, y) in the Coordinate Plane
  • Determine the x-coordinate. Beginning at the origin, move horizontally, the direction of the x-axis, the distance given by the x-coordinate. If the x-coordinate is positive, move to the right; if the x-coordinate is negative, move to the left.
  • Determine the y-coordinate. Beginning at the x-coordinate, move vertically, the direction of the y-axis, the distance given by the y-coordinate. If the y-coordinate is positive, move up; if the y-coordinate is negative, move down.
  • Draw a point at the ending location. Label the point with the ordered pair.
  • An ordered pair is represented by a single point on the graph.
• Sign Patterns of the Quadrants
  

  Quadrant I	Quadrant II	Quadrant III	Quadrant IV
  [latex](x,y)[/latex]	[latex](x,y)[/latex]	[latex](x,y)[/latex]	[latex](x,y)[/latex]
  [latex](+,+)[/latex]	[latex](−,+)[/latex]	[latex](−,−)[/latex]	[latex](+,−)[/latex]

• Coordinates of Zero
  • Points with a [latex]y[/latex]-coordinate equal to [latex]0[/latex] are on the x-axis, and have coordinates [latex](a, 0)[/latex].
  • Points with a [latex]x[/latex]-coordinate equal to [latex]0[/latex] are on the y-axis, and have coordinates [latex](0, b)[/latex].
  • The point [latex](0, 0)[/latex] is called the origin. It is the point where the x-axis and y-axis intersect.

• Identifying Solutions  To find out whether an ordered pair is a solution of a linear equation, you can do the following:

  • Graph the linear equation, and graph the ordered pair. If the ordered pair appears to be on the graph of a line, then it is a possible solution of the linear equation. If the ordered pair does not lie on the graph of a line, then it is not a solution.
  • Substitute the (x, y) values into the equation. If the equation yields a true statement, then the ordered pair is a solution of the linear equation. If the ordered pair does not yield a true statement then it is not a solution.

Glossary

linear equation

An equation of the form [latex]Ax+By=C[/latex], where [latex]A[/latex] and [latex]B[/latex] are not both zero, is called a linear equation in two variables.

ordered pair

An ordered pair [latex]\left(x,y\right)[/latex] gives the coordinates of a point in a rectangular coordinate system. The first number is the [latex]x[/latex] -coordinate. The second number is the [latex]y[/latex] -coordinate.

[latex]\underset{x\text{-coordinate},y\text{-coordinate}}{\left(x,y\right)}[/latex]

origin

The point [latex]\left(0,0\right)[/latex] is called the origin. It is the point where the the point where the [latex]x[/latex] -axis and [latex]y[/latex] -axis intersect.

quadrants

The [latex]x[/latex] -axis and [latex]y[/latex] -axis divide a rectangular coordinate system into four areas, called quadrants.  The quadrants are labeled with the Roman Numerals I, II, III, IV going around the coordinate system in a counter-clockwise direction.

solution to a linear equation in two variables

An ordered pair [latex]\left(x,y\right)[/latex] is a solution to the linear equation [latex]Ax+By=C[/latex], if the equation is a true statement when the x- and y-values of the ordered pair are substituted into the equation.

x-axis

The x-axis is the horizontal axis in a rectangular coordinate system.

y-axis

The y-axis is the vertical axis on a rectangular coordinate system.