Areas of Polygons for 7th Grade

🏆Practice area of a rectangle

Areas of Polygons

Polygon Definition

A polygon defines a geometric shape that is made up of sides. In other words, under the umbrella of polygons fall the following square, rectangle, parallelogram, trapezoid, and many more.

For example, a triangle has 3 sides, every quadrilateral has 4 sides, and so on.

We have already learned to calculate the areas of standard polygons. There are also non-standard polygons, for which there is no specific formula. However, their area of complex shapes can be calculated using two methods:

  • We can divide the area of the required polygon into several areas of polygons that we are familiar with, calculate the areas separately, and then add them together to obtain the final area.
  • We can try to "complete" the area of the required polygon into another polygon whose area we know how to calculate, and the proceed to subtract the area we added. This way, we can obtain the area of the original polygon.

Example

Let's demonstrate this using a simple exercise:

Diagram of a composite shape divided into two rectangles, with dimensions labeled. The left rectangle has dimensions 7 by 4 with an area (A) of 28, and the right rectangle has dimensions 3 by 6 with an area (A) of 18. The diagram illustrates how to calculate areas of composite polygons by dividing them into simpler shapes. Featured in a tutorial on calculating areas of polygons.

Here is a drawing of a polygon.

We need to calculate its area. From the start, we can see that this is not a standard polygon, so we will use the first method to calculate its area. We will divide the polygon as shown in the drawing, and we should obtain two rectangles.

According to the data shown in the drawing, in the rectangle on the right side we obtain the side lengths of 3 and 6, therefore the area of the rectangle will be 18 (multiplication of the two values). In the rectangle on the left side we obtain the side lengths of 4 and 7, therefore the area of the rectangle will be 28 (multiplication of the two values). Thus, the total area of the polygon will be the sum of the two areas we calculated separately, meaning, 18+28=46.

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Test yourself on area of a rectangle!

Complete the sentence:

To find the area of a right triangle, one must multiply ________________ by each other and divide by 2.

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In 7th grade we focus on learning about several polygons (click on the links for in-depth reading):

How to calculate areas of polygons

The formula for calculating the area of a polygon varies according to the polygon in question. (Click on the titles to read the full articles including examples and practice)

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Test your knowledge

Calculating Rectangle Area

The formula for calculating the area of a rectangle is: width X length.

S=whS=w\cdot h

A2 - Rectangle area formula

Calculating the area of any triangle

The formula for calculating the area of any triangle: base X height divided by 2

S=Baseh2S={{Base \cdot h}\over 2}

A3 - Triangle Area Formula

Do you know what the answer is?

Calculating the area of a right triangle

In the case of a right triangle's area, it's the same formula, but the height is actually one of the sides

A - area of a new right triangle

Calculating the Area of a Parallelogram

The area of a parallelogram is calculated by multiplying one of its sides by the height.

S=Baseh2S={{Base \cdot h}\over 2}

For example in the drawing, you can calculate the area of the parallelogram by multiplying DC by h1 and then dividing by 2, or by multiplying BC by h2 and then dividing by 2

1 - Area of a Parallelogram

Check your understanding

Calculating the Area of a Trapezoid

The formula for calculating the area of a trapezoid is the sum of the two bases X the height divided by 2

S=(Base1+Base2)h2S={{(Base{_1} + Base{_2}) \cdot h}\over 2}

A1 - How do you calculate the area of a new trapezoid

Do you think you will be able to solve it?

Examples with solutions for Areas of Polygons for 7th Grade

Exercise #1

AB = 10 cm

The height of the rectangle is 5 cm.

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Calculate the area of the parallelogram.

Video Solution

Step-by-Step Solution

To solve this problem, we'll apply the formula for the area of a parallelogram:

  • Step 1: Identify the base and the height from the given information.
  • Step 2: Use the formula for the area of a parallelogram: A=base×height A = \text{base} \times \text{height} .
  • Step 3: Calculate the area using the given values.

Let's proceed with the solution:
Step 1: The given base AB AB is 10 cm, and the height is 5 cm.
Step 2: The formula for the area of a parallelogram is A=base×height A = \text{base} \times \text{height} .
Step 3: Substituting the provided values, we get:
A=10cm×5cm A = 10 \, \text{cm} \times 5 \, \text{cm}
A=50cm2 A = 50 \, \text{cm}^2

Therefore, the area of the parallelogram is 50cm2 50 \, \text{cm}^2 .

Answer

50

Exercise #2

Calculate the area of the following parallelogram:

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Video Solution

Step-by-Step Solution

To calculate the area of the parallelogram, we will simply apply the formula for the area of a parallelogram:

  • Identify the base: The length of the base is 10cm10 \, \text{cm}.
  • Identify the height: The perpendicular height is given as 6cm6 \, \text{cm}.

Apply the formula: Area=base×height \text{Area} = \text{base} \times \text{height} .

Substitute the known values: Area=10cm×6cm \text{Area} = 10 \, \text{cm} \times 6 \, \text{cm} .

Calculate the result: Area=60cm2 \text{Area} = 60 \, \text{cm}^2 .

Therefore, the area of the parallelogram is 60cm2 60 \, \text{cm}^2 .

Answer

60 cm²

Exercise #3

AB = 12 cm

The height of the rectangle is 4 cm.

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Calculate the area of the parallelogram.

Video Solution

Step-by-Step Solution

To solve this problem, we'll proceed as follows:

  • Step 1: Identify the given values for the base and the height of the parallelogram.
  • Step 2: Apply the formula for calculating the area of the parallelogram.
  • Step 3: Calculate the area using the values provided.

Let's perform each step:

Step 1: From the problem, we know:

  • The base AB AB of the parallelogram is 12cm 12 \, \text{cm} .
  • The height relative to the base is 4cm 4 \, \text{cm} .

Step 2: Use the formula for the area of a parallelogram:

Area=base×height\text{Area} = \text{base} \times \text{height}

Step 3: Plugging in the values of the base and height:

Area=12×4=48cm2\text{Area} = 12 \times 4 = 48 \, \text{cm}^2

Therefore, the area of the parallelogram is 48cm2 48 \, \text{cm}^2 .

Since this is a multiple-choice problem, the correct answer is Choice 2.

Answer

48

Exercise #4

AB = 15 cm

The height of the rectangle is 6 cm.

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Calculate the area of the parallelogram.

Video Solution

Step-by-Step Solution

To solve this problem, we'll follow these steps:

  • Step 1: Identify the given base and height.
  • Step 2: Apply the formula for the area of a parallelogram.
  • Step 3: Calculate the area using the provided dimensions.

Now, let's work through each step:
Step 1: The base b b is equal to the length AB AB , which is 15 cm\text{15 cm}. The height h h corresponding to this base is 6 cm\text{6 cm}.
Step 2: We'll use the formula for the area of a parallelogram:
Area=b×h\text{Area} = b \times h.
Step 3: Plugging in our values, we have:
Area=15×6=90cm2\text{Area} = 15 \times 6 = 90 \, \text{cm}^2.

Therefore, the solution to the problem is Area=90cm2 \text{Area} = 90 \, \text{cm}^2 , which matches choice .

Answer

90

Exercise #5

AB = 17 cm

The height of the rectangle is 8 cm.

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Calculate the area of the parallelogram.

Video Solution

Step-by-Step Solution

To solve this problem, we will calculate the area of the parallelogram using the given base and height dimensions.

  • Step 1: Identify the given parameters. The base of the parallelogram AB=17cm AB = 17 \, \text{cm} and the corresponding height is 8cm 8 \, \text{cm} .
  • Step 2: Apply the area formula for parallelograms: Area=base×height\text{Area} = \text{base} \times \text{height}.
  • Step 3: Substitute the given values into the formula: Area=17×8 \text{Area} = 17 \times 8 .

Calculating the product, we have:
Area=136cm2 \text{Area} = 136 \, \text{cm}^2 .

Therefore, the area of the parallelogram is 136cm2 136 \, \text{cm}^2 .

Answer

136

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