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A number line helps visualize one-dimensional quantities like temperature and time. However, there are situations where two dimensions are needed, such as when latitude and longitude when locating places on a map. The coordinate plane is used in these situations! This lesson will teach how to locate points on a coordinate plane and explore some of its uses.

Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

Explore

The Battleship Game Board

Get ready for an adventure! Discover the location of enemy ships hidden on the game board by using specific points. Each point on the board is represented by the combination of a letter and a number. Place the boats by dragging them from the stern, or the back, and rotate them by clicking on the bow, or the front.
The Battleship Game Board
External credits: Freepik
Consider the following questions.
  • How does using the letter and number combination to locate enemy ships in the game relate to finding a location on a map in real life?
  • What are other situations where two numbers, two letters, or a combination of both help to indicate a location?
Discussion

Coordinate System

A coordinate system is a reference framework used to describe the positions of objects like points, lines, and surfaces in a space. A fixed point called the origin is used as a reference in the coordinate system. The most common types of coordinate systems are one-, two-, or three-dimensional.

Coordinate Plane

A coordinate plane is a two-dimensional coordinate system. It is a grid that results from intersecting a vertical number line with a horizontal number line at their zero points. The horizontal number line is usually named the axis and the vertical number line is usually the axis.

A figure showing that a coordinate plane has two perpendicular axes: the horizontal x-axis and the vertical y-axis. They intersect at the origin.
The positive numbers on the axis are above zero and the negative ones are below zero. The origin is where the lines intersect, which is the point
Discussion

Coordinate

A coordinate is the position of an object in a coordinate system relative to the corresponding axis. Coordinates are often seen together with other coordinates, which can describe the position of an object in a coordinate system with one, two, three, or even more dimensions.

In Two Dimensions

In a two-dimensional coordinate system, points are usually expressed as a coordinate pair — also called an ordered pair — denoted by The first coordinate states the position along the axis and the second coordinate states the position along the axis.

Point on the 2D coordinate plane
Pop Quiz

Random Points on a Coordinate System

Identify the coordinates of the given point. Write the coordinates as an ordered pair where represents the coordinate and the coordinate.

Random point on a coordinate plane.
Example

The Path From Home to the Basketball Court

Dominika plays basketball with her friends every afternoon. She warms up by running and picks up two friends, Jordan and Emily, on the way to the court. Dominika uses ordered pairs to represent the positions of her and her friends' houses.

Position
Dominika's House
Jordan's House
Emily's House
a Dominika wants to display this information on a coordinate plane. Which graph correctly displays the positions of the three houses?
Four coordinate planes
b The basketball court is at the point , and Dominika's whole path from her house to the court and back can be represented by joining points D, J, E, C, and D. Which graph shows Dominika's whole path?
Four coordinate planes

Hint

a An ordered pair can be graphed on a coordinate plane by moving horizontally from the origin the number of units specified in its coordinate and then vertically the number of units specified in its coordinate.
b Plot the coordinates of the basketball court. Connect the points using line segments in the given order.

Solution

a The positions of the girls' houses are given as ordered pairs. Recall that an ordered pair can be graphed on a coordinate plane by moving horizontally from the origin the number of units specified in its coordinate and vertically the number of units specified in its coordinate.
How to graph an ordered pair.
With this information in mind, consider the position of Dominika's house.
The coordinate is and the coordinate is Move units to the right of the origin and then unit up to graph Dominika's house on the coordinate plane.
Dominika's house position on a coordinate plane.
Now, consider the coordinates of Jordan's and Emily's houses.
On the coordinate plane, move units horizontally to the right of the origin and then units up to graph the position of Jordan's house. To plot Emily's house, start at the origin again. Move unit to the left and units up to graph Emily's house.
Girl's house positions.
Notice that this corresponds to graph III.
b Begin by considering the coordinates of the position of the basketball court.
Add this point to the graph from the previous part. Move unit horizontally to the left of the origin and then unit up.
Point F added to the graph of Girl's house positions.
The points can now be joined according to the given order D, J, E, F, and D. Draw a line starting from point D to point J. Next, from point J, go to point E, and so on until point C is connected to point D.
A square drawn from the points representing the field and the girl's houses positions.
This graph matches the one given in option I.
Example

Dominika's Running Routine

Dominika's house is represented by the point on a coordinate plane. Every morning, she runs to a place five blocks away from her house, then returns home using the same route.
Dominika's location on a coordinate plane.
If Dominika picks a different spot to visit every day, which graph displays all the places she could go? Consider that every unit on the coordinate plane represents one block and that Dominika stays on the straight roads.
Join all the potential places Dominika could go. What shape do these points form?

Hint

Moving one unit on the axis or axis on the coordinate plane is counted as moving one block. Since Dominika can only stay on the straight roads, her position can only move up, down, left, and right on the graph.

Solution

Moving one unit on the axis or axis on the coordinate plane is the same as Dominika moving one block. Dominika only runs on the straight roads, so her progress on the graph can only move in four directions — up, down, left, and right.

Allowed movements

Keeping this in mind, one route she can take is moving five blocks directly to the right. Move units to the right starting her house to show this possible destination. Remember that Dominika's house is located at not the origin!

Showing the point (7,1) on a coordinate plane as a possible resting point.

Another spot Dominika might go is one block up and four blocks to the right of her starting point. It does not matter if she moves up on her first, second, third, fourth, or final move. Her destination will always be the same if she moves one block up out of the five total moves.

Showing (6,1) as a possible resting point on a coordinate plane.

Follow the same reasoning to find the other places that Dominika can go. Plot each as ordered pairs on the coordinate plane.

All the ordered pairs graphed on a coordinate plane.

Now that all the points are plotted on the graph, connect them with lines to see the overall shape.

All ordered pairs connected with lines to form a square.

The shape is a rotated square.

Showing Our Work

Finding All the Places Dominika Can Go

The places Dominika can go can be determined by identifying all the possible routes she can take. Remember, she can only go five blocks away from her home. Moving left or down indicates a negative direction on the corresponding axis, represented by negative numbers.

Route from Home Calculations Destination
blocks to the right
blocks to the right and block up
blocks to the right and blocks up
blocks to the right and blocks up
block to the right and blocks up
blocks up
block to the left and blocks up
blocks to the left and blocks up
blocks to the left and blocks up
blocks to the left and block up
blocks to the left
blocks to the left and block down
blocks to the left and blocks down
blocks to the left and blocks down
block to the left and blocks down
blocks down
block to the right and blocks down
blocks to the right and blocks down
blocks to the right and blocks down
blocks to the right and block down
Discussion

Quadrants

In a coordinate plane, the intersection of the axis and the axis produces four regions called quadrants. The quadrants are numbered counterclockwise from the top right quadrant as Quadrant I to Quadrant IV in the bottom right.

Quadrants labeled on a coordinate plane.

The signs of the coordinates of a point can be determined based on which quadrant the point lies in.

Quadrants labeled on a coordinate plane.
Conversely, the quadrant of a point can be found by looking at the signs of its coordinates.
Points on an axis do not belong to any quadrant.
Pop Quiz

Finding the Quadrant in Which a Point Lies

A coordinate system has four quadrants that provide information about the signs of the and coordinates of a point. In the following applet, identify in which quadrant the given point lies.

Example

Dominika is drafting a logo for her basketball team to wear on their jerseys. She wants the logo to have a cool geometric shape, so she sets some points that she thinks will look good. Help her design the logo by matching each point with its corresponding quadrant on the coordinate plane.

Hint

Look at the signs of the coordinates of each point to identify their quadrants.

Solution

The quadrant in which a point lies can be determined by looking at the signs of its coordinates. For instance, a point in Quadrant I has a positive coordinate and a positive coordinate. Consider the given points and see if any of them fit this description.
The point is located in Quadrant I because both of its coordinates are positive. In contrast, a point in Quadrant II has a negative coordinate and a positive coordinate. Look at the remaining points to see which one fits this description.
The point is in Quadrant II because its coordinate is negative but its coordinate is positive. Follow a similar reasoning to find the corresponding quadrants of the remaining points.
Point Signs of Coordinates Quadrant
Quadrant III
Quadrant IV
Quadrant II
Quadrant I
Now that the quadrant of each point has been identified, plot the points on a coordinate plane. Move horizontally from the origin the number of units indicated by the coordinate and vertically the number of units specified by the coordinate of each point.
Points plotted on a coordinate plane.
External credits: Image by Freepik
The logo is a hit, and the team gains more and more fans as a result!
Discussion

Reflecting a Point Across an Axis on the Coordinate Plane

Reflecting a point means making a mirror image of that point by flipping it across a certain line or axis. The following rules show how to find reflected points across a specific axis.

Point Axis of Reflection Procedure Reflected Point
Change the coordinate to its opposite.
Change the coordinate to its opposite.
and Change the and coordinates to their opposites.

Next, the process for reflecting points across the axis, the axis, or both, on a coordinate plane will be shown. Consider the points and

Point Axis of Reflection Reflected Point
 ?
 ?
and  ?
There are two steps to follow to reflect a point.
1
Find the Reflected Point
expand_more
Keep the same coordinate of a point and change the coordinate to its opposite to reflect it across the axis. With this information, point can be reflected across the axis by changing its coordinate to its opposite,
In contrast, change the coordinate of a point to its opposite and keep its coordinate to reflect it across the axis. Point can be reflected across the axis by changing the coordinate to its opposite,
Finally, change the coordinate and the coordinate of a point to their opposites to reflect it across both the axis and axis. Therefore, reflect point across both axes by changing its coordinate from to and its coordinate from to


2
Plot the Point and Its Reflection on the Coordinate Plane
expand_more

Once each initial point and its reflection across a given axis are found, plot both points on the coordinate plane.

Each point and its reflection plotted on a coordinate plane.
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