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Connecting Algebra and Geometry Through Coordinates

Perimeter and Area in a Coordinate Plane

On a coordinate plane, each point has coordinates. The coordinates of a line segment's endpoints can be used to calculate the line segment's length. In this lesson, the perimeter and area of different polygons will be calculated using the vertices' coordinates.

Catch-Up and Review

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

Try your knowledge on these topics.

a State the coordinates of the points plotted on the plane.
Points on a coordinate plane
b Determine whether a triangle with side lengths 4, 5, and 6 is a right triangle.
c Use the Pythagorean Theorem to find the missing side length. Round the answer to one decimal place.
A right triangle with given side lengths
d Calculate MN by using the Distance Formula.
Segment MN on a coordinate plane
e Find the perimeter of the given polygon.
A polygon with the given side lengths
f Calculate the area of the rectangle.
A rectangle with the given width and length

Challenge

Investigate Shapes by Using Coordinates

The coordinates of the vertices of a polygon can be used to identify the type of the polygon.

Emily's sister completed her first 1-mile run. Emily wants to make a congratulatory card in the shape of a triangle for her. She wants the card to have the form of an equilateral triangle. However, she lost her ruler and made all the measurements by eyeballing it.

A triangular card
How can Emily determine if the card has the shape of an equilateral, an isosceles, or a scalene triangle?

Example

Solving Problems in the Real World by Using Coordinates

Izabella wants to buy a plot of land for her garden. This rectangular plot of land should have a minimum area of 30 square meters and a perimeter no less than 20 meters to have enough space. She knows the coordinates of the vertices of the plot of land.

A rectangular plot of land on the grass

Does this plot of land satisfy Izabella's requirements?

Hint

Use the Distance Formula to find the width and the length of the rectangle.

Solution

The area and perimeter of a rectangle can be calculated using the following formulas.
Here, w and are the width and the length of the rectangle. In the diagram, the coordinates of the vertices are given. Using the Distance Formula, the length and width of a rectangle can be calculated.
First, the and of the rectangle need to be identified.
The rectangular plot of land with the identified length and width

Next, by substituting the endpoints of the chosen segments into the Distance Formula, their lengths can be calculated.

Length Width
Endpoints and (3,5) (3,5) and (5,0)
Substitute
Evaluate

Now that the length and the width are known, the area and the perimeter of the rectangle can be determined.

Area Perimeter
Formula
Substitute A=5.4(8.5) P=2(5.4+8.5)
Evaluate A=45.9 P=27.8

Since these values are greater than the area and the perimeter of the plot of land that Izabella wanted to buy, they satisfy the given requirements. Therefore, Izabella should buy this plot of land.

Example

Investigate the Properties of a Triangle

On a test, Kriz is asked to find the area and perimeter of a triangle illustrated on a coordinate plane.

A triangle on a coordinate plane

Kriz wrote that the area of MNK is 15 square units and its perimeter is 24 units. Is Kriz correct?

Hint

Find the coordinates of the vertices using the coordinate plane. Then use the Distance Formula to calculate the side lengths of the triangle.

Solution

First, the coordinates of the vertices on the coordinate plane need to be noted.
A triangle on a coordinate plane with the coordinates of the vertices
Next, the side lengths of MNK need to be calculated by using the Distance Formula. The calculations for MN are shown below.
Evaluate right-hand side
By following the same procedure, NK and MK can be calculated.
Side Endpoints Substitute Evaluate
MN and N(3,4)
NK N(3,4) and
MK and MK=6
Now, the perimeter of MNK can be calculated by adding the three side lengths of the triangle.
The perimeter of MNK is 19.9 units. Next, its area will be found. Recall that the area of a triangle is half the product of the base and its corresponding height.
In this formula, b is the base of the triangle and h its corresponding height. In a triangle, the base and its corresponding height are perpendicular. If MK is the base, then the height is the perpendicular segment to MK through N. In the diagram, it can be seen that the length of this segment is 6 units.
A height drawn to one side of the triangle
Finally, by substituting b=6 and h=6, the area of MNK can be calculated.
Evaluate right-hand side
A=18
Therefore, the area of the triangle is 18 square units and its perimeter 19.9 units. This means that Kriz was not correct.

Example

Investigate the Properties of a Compound Shape

The plan of a flat is represented on a coordinate plane. The plan has the shape of a compound geometric figure.

Plan of a flat on a coordinate plane

Calculate the area and the perimeter of the plan. Round each value to one decimal place.

Hint

Use the coordinate plane to find the side lengths of the compound figure. The area of a compound figure is equal to the sum of the areas of the geometric figures that make the compound figure.

Solution

Start by identifying the coordinates of the vertices and the lengths of the horizontal and vertical sides of the figure. Let m be the length of the side which is neither vertical nor horizontal.
Plan of a flat on a coordinate plane
The value of m can be calculated by substituting (-5,1) and (0,4) into the Distance Formula.
Evaluate right-hand side
By adding the lengths of the exterior sides, the perimeter of the plan can be calculated.
The perimeter of the figure is 31.8 units. The area of a compound figure is equal to the sum of the geometric figures' areas that make the compound figure. From the diagram, it can be seen that the flat has a square, a rectangular, and a triangular room.

Square Room

The area of a square is equal to the square of a side length. It has been previously found that, in the plan, each side of the square room is 6 units long.
Square room highlighted
By raising 6 to the power of two, the area of the square can be determined.

Rectangular Room

The area of a rectangle is equal to the product of its length and width. From the diagram, it can be seen that the length of the rectangle is 5 units and the width is 3 units.
Rectangular room highlighted
By multiplying 3 by 5, the area of the rectangle can be found.

Triangular Room

The area of a triangle is half the product of its base and its height. Analyzing the diagram, it can be seen that the base and height of the triangle are b=5 units and h=3 units, respectively.
Triangular room highlighted
By substituting these values into the formula for the area of a triangle, its area can be calculated.
Evaluate right-hand side
A=7.5

Total Area

Gather all the areas that have been found.
By adding these values, the area of the compound figure can be calculated.

Example

Investigate the Properties of a Triangle

The diagram illustrates a triangle on a coordinate plane, and gives the coordinates of the vertices.

A triangle on a coordinate plane with the coordinates of the vertices
Calculate the area of the triangle. If needed, round the answer to one decimal place.

Hint

Use the formula where s is the semi-perimeter, and a, b, and c are the side lengths of the triangle.

Solution

To calculate the area of a triangle, Heron's formula can be used.
Here, a, b, and c are the side lengths and s is the semi-perimeter of the triangle. The semi-perimeter is half the perimeter.
In the diagram, the coordinates of the vertices are given. The sides of the triangle will be labeled as a, b, and c.
A triangle on a coordinate plane with the coordinates of the vertices
By substituting the coordinates of the vertices into the Distance Formula, the side lengths can be calculated.
Side Endpoints Substitute Evaluate
a (2,3) and (3,7.5)
b (3,7.5) and (7,1)
c (2,3) and (7,1)
Next, the side lengths will be substituted into the formula for the semi-perimeter to find the value of s.
Evaluate right-hand side
s=8.8
Now that the semi-perimeter of the triangle is known, its area can be found using Heron's Formula.
Evaluate right-hand side
A=150.7968
The area of the triangle is approximately 150.8 square units.

Closure

Classifying Triangles by Using Coordinates

With the topics seen in this lesson, the challenge presented at the beginning can be solved. It consisted in determining whether the card made by Emily has the shape of an equilateral, an isosceles, or a scalene triangle.

A triangular card

Hint

Plot the card on a coordinate plane so that one of its vertices is at the origin. Identify the coordinates of the vertices and then use them to calculate the side lengths.

Solution

The type of triangle can be identified by comparing the side lengths. In order to find them, the coordinates of the vertices should be known. Plot the card on a coordinate plane so that one of its vertices is at the origin.
Emily's card on a coordinate plane
Note that the bottom side is a horizontal segment. Therefore, its length is equal to the difference of the x-coordinates of the endpoints.
The lengths of the other two sides can be calculated using the Distance Formula.
Side Endpoints Substitute Evaluate
AB A(0,0) and B(4,7)
BC B(4,7) and C(8,0)

As it can be seen, AB and BC have the same length. However, they are a little longer than AC. Therefore, Emily's card has a shape of an isosceles triangle.

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