Intermediate Geometry : How to find the area of an equilateral triangle

Study concepts, example questions & explanations for Intermediate Geometry

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Example Questions

Example Question #231 : Triangles

Find the area of an equilateral triangle with a perimeter of 24cm. Leave answer in simplest radical form. 

Possible Answers:

\displaystyle 64\ cm^{2}

\displaystyle 16\sqrt3\ cm^{2}

\displaystyle 32\sqrt3\ cm^{2}

\displaystyle 32\ cm^{2}

Correct answer:

\displaystyle 16\sqrt3\ cm^{2}

Explanation:

To find the area of an equilateral triangle, one must find the base and the height. 

\displaystyle Area=\frac{1}{2}bh

All the sides of an equilateral triangle are congruent, so if the perimeter of the equilaterail triangle is 24, then each side must equal one third of that total which is 8cm. 

This will produce a triangle that includes the following information below: 

 

1 ans2

Dropping an altitude down the center of the equilateral triangle will result in two 30-60-90 triangles with a hypotenuse of 8. 

1 ans 3

In every 30-60-90 triangle the following formulas apply: 

\displaystyle hypotenuse=2(shortleg)

\displaystyle longleg=shortleg\sqrt3

When we plug in the given information on the triangle we get: 

\displaystyle 8=2(shortleg)

Dividing both sides by 2 gives the below result.

\displaystyle shortleg=4

We can now plug this into the long leg formula to get the height of the triangle: 

\displaystyle longleg=4\sqrt3

 

1 anspng

Now that we have all of the information needed to find the area we plug these values into the area formula. 

\displaystyle Area=\frac{1}{2}bh

\displaystyle Area=\frac{1}{2}(8)(4\sqrt3)

\displaystyle Area=16\sqrt3

Example Question #31 : Equilateral Triangles

A circle with a radius of \displaystyle 2 is inscribed in an equilateral triangle with side lengths of \displaystyle 6 as shown in the figure below.

1

Find the area of the shaded region.

Possible Answers:

\displaystyle 3.68

\displaystyle 2.01

\displaystyle 1.99

\displaystyle 3.02

Correct answer:

\displaystyle 3.02

Explanation:

In order to find the area of the shaded region, we must first find the area of the circle and the area of the equilateral triangle.

Recall how to find the area of a circle:

\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2

Plug in the given radius to find the area of the circle.

\displaystyle \text{Area of Circle}=\pi\times 2^2=4\pi

Next, recall how to find the area of an equilateral triangle:

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(\text{side})^2

Plug in the length of the side of the triangle to find the area.

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(6)^2=9\sqrt3

In order to find the area of the shaded region, we will need to subtract the area of the circle from the area of the triangle.

\displaystyle \text{Area of Shaded Region}=\text{Area of Triangle}-\text{Area of Circle}

\displaystyle \text{Area of Shaded Region}=9\sqrt3-4\pi=3.02

Make sure to round to \displaystyle 2 places after the decimal.

Example Question #11 : How To Find The Area Of An Equilateral Triangle

A circle with a radius of \displaystyle 3 is inscribed in an equilateral triangle with side lengths of \displaystyle 9 as shown in the figure below.

1

Find the area of the shaded region.

Possible Answers:

\displaystyle 6.33

\displaystyle 7.25

\displaystyle 5.87

\displaystyle 6.80

Correct answer:

\displaystyle 6.80

Explanation:

In order to find the area of the shaded region, we must first find the area of the circle and the area of the equilateral triangle.

Recall how to find the area of a circle:

\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2

Plug in the given radius to find the area of the circle.

\displaystyle \text{Area of Circle}=\pi\times 3^2=9\pi

Next, recall how to find the area of an equilateral triangle:

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(\text{side})^2

Plug in the length of the side of the triangle to find the area.

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(9)^2=\frac{81\sqrt3}{4}

In order to find the area of the shaded region, we will need to subtract the area of the circle from the area of the triangle.

\displaystyle \text{Area of Shaded Region}=\text{Area of Triangle}-\text{Area of Circle}

\displaystyle \text{Area of Shaded Region}=\frac{81\sqrt3}{4}-9\pi=6.80

Make sure to round to \displaystyle 2 places after the decimal.

Example Question #13 : How To Find The Area Of An Equilateral Triangle

A circle with a radius of \displaystyle 4 is inscribed in an equilateral triangle with side lengths of \displaystyle 12 as shown in the figure below.

1

Find the area of the shaded region.

Possible Answers:

\displaystyle 13.08

\displaystyle 10.27

\displaystyle 12.09

\displaystyle 16.33

Correct answer:

\displaystyle 12.09

Explanation:

In order to find the area of the shaded region, we must first find the area of the circle and the area of the equilateral triangle.

Recall how to find the area of a circle:

\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2

Plug in the given radius to find the area of the circle.

\displaystyle \text{Area of Circle}=\pi\times 4^2=16\pi

Next, recall how to find the area of an equilateral triangle:

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(\text{side})^2

Plug in the length of the side of the triangle to find the area.

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(12)^2=36\sqrt3

In order to find the area of the shaded region, we will need to subtract the area of the circle from the area of the triangle.

\displaystyle \text{Area of Shaded Region}=\text{Area of Triangle}-\text{Area of Circle}

\displaystyle \text{Area of Shaded Region}=36\sqrt3-16\pi=12.09

Make sure to round to \displaystyle 2 places after the decimal.

Example Question #14 : How To Find The Area Of An Equilateral Triangle

A circle with a radius of \displaystyle 5 is inscribed in an equilateral triangle with side lengths of \displaystyle 20 as shown in the figure below.

1

Find the area of the shaded region.

Possible Answers:

\displaystyle 100.36

\displaystyle 99.07

\displaystyle 97.86

\displaystyle 94.67

Correct answer:

\displaystyle 94.67

Explanation:

In order to find the area of the shaded region, we must first find the area of the circle and the area of the equilateral triangle.

Recall how to find the area of a circle:

\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2

Plug in the given radius to find the area of the circle.

\displaystyle \text{Area of Circle}=\pi\times 5^2=25\pi

Next, recall how to find the area of an equilateral triangle:

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(\text{side})^2

Plug in the length of the side of the triangle to find the area.

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(20)^2=100\sqrt3

In order to find the area of the shaded region, we will need to subtract the area of the circle from the area of the triangle.

\displaystyle \text{Area of Shaded Region}=\text{Area of Triangle}-\text{Area of Circle}

\displaystyle \text{Area of Shaded Region}=100\sqrt3-25\pi=94.67

Make sure to round to \displaystyle 2 places after the decimal.

Example Question #681 : Intermediate Geometry

A circle with a radius of \displaystyle 6 is inscribed in an equilateral triangle with side lengths of \displaystyle 24 as shown in the figure below.

1

Find the area of the shaded region.

Possible Answers:

\displaystyle 132.02

\displaystyle 128.95

\displaystyle 136.32

\displaystyle 120.14

Correct answer:

\displaystyle 136.32

Explanation:

In order to find the area of the shaded region, we must first find the area of the circle and the area of the equilateral triangle.

Recall how to find the area of a circle:

\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2

Plug in the given radius to find the area of the circle.

\displaystyle \text{Area of Circle}=\pi\times 6^2=36\pi

Next, recall how to find the area of an equilateral triangle:

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(\text{side})^2

Plug in the length of the side of the triangle to find the area.

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(24)^2=144\sqrt3

In order to find the area of the shaded region, we will need to subtract the area of the circle from the area of the triangle.

\displaystyle \text{Area of Shaded Region}=\text{Area of Triangle}-\text{Area of Circle}

\displaystyle \text{Area of Shaded Region}=144\sqrt3-36\pi=136.32

Make sure to round to \displaystyle 2 places after the decimal.

Example Question #11 : How To Find The Area Of An Equilateral Triangle

A circle with a radius of \displaystyle 6 is inscribed in an equilateral triangle with side lengths of \displaystyle 20 as shown by the figure below.

1

Find the area of the shaded region.

Possible Answers:

\displaystyle 60.11

\displaystyle 51.28

\displaystyle 62.39

\displaystyle 55.22

Correct answer:

\displaystyle 60.11

Explanation:

In order to find the area of the shaded region, we must first find the area of the circle and the area of the equilateral triangle.

Recall how to find the area of a circle:

\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2

Plug in the given radius to find the area of the circle.

\displaystyle \text{Area of Circle}=\pi\times 6^2=36\pi

Next, recall how to find the area of an equilateral triangle:

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(\text{side})^2

Plug in the length of the side of the triangle to find the area.

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(20)^2=100\sqrt3

In order to find the area of the shaded region, we will need to subtract the area of the circle from the area of the triangle.

\displaystyle \text{Area of Shaded Region}=\text{Area of Triangle}-\text{Area of Circle}

\displaystyle \text{Area of Shaded Region}=100\sqrt3-36\pi=60.11

Make sure to round to \displaystyle 2 places after the decimal.

Example Question #241 : Triangles

A circle with a radius of \displaystyle 3 is inscribed in an equilateral triangle with side lengths of \displaystyle 14 as shown by the figure below.

1

Find the area of the shaded region.

Possible Answers:

\displaystyle 56.60

\displaystyle 52.20

\displaystyle 58.90

\displaystyle 52.31

Correct answer:

\displaystyle 56.60

Explanation:

In order to find the area of the shaded region, we must first find the area of the circle and the area of the equilateral triangle.

Recall how to find the area of a circle:

\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2

Plug in the given radius to find the area of the circle.

\displaystyle \text{Area of Circle}=\pi\times 3^2=9\pi

Next, recall how to find the area of an equilateral triangle:

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(\text{side})^2

Plug in the length of the side of the triangle to find the area.

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(14)^2=49\sqrt3

In order to find the area of the shaded region, we will need to subtract the area of the circle from the area of the triangle.

\displaystyle \text{Area of Shaded Region}=\text{Area of Triangle}-\text{Area of Circle}

\displaystyle \text{Area of Shaded Region}=49\sqrt3-9\pi=56.60

Make sure to round to \displaystyle 2 places after the decimal.

Example Question #242 : Triangles

A circle with a radius of \displaystyle 12 is inscribed in an equilateral triangle with side lengths of \displaystyle 40 as shown by the figure below.

1

Find the area of the shaded region.

Possible Answers:

\displaystyle 240.43

\displaystyle 256.31

\displaystyle 199.68

\displaystyle 225.31

Correct answer:

\displaystyle 240.43

Explanation:

In order to find the area of the shaded region, we must first find the area of the circle and the area of the equilateral triangle.

Recall how to find the area of a circle:

\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2

Plug in the given radius to find the area of the circle.

\displaystyle \text{Area of Circle}=\pi\times 12^2=144\pi

Next, recall how to find the area of an equilateral triangle:

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(\text{side})^2

Plug in the length of the side of the triangle to find the area.

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(40)^2=400\sqrt3

In order to find the area of the shaded region, we will need to subtract the area of the circle from the area of the triangle.

\displaystyle \text{Area of Shaded Region}=\text{Area of Triangle}-\text{Area of Circle}

\displaystyle \text{Area of Shaded Region}=400\sqrt3-144\pi=240.43

Make sure to round to \displaystyle 2 places after the decimal.

Example Question #243 : Triangles

A circle with a radius of \displaystyle \frac{1}{2} is inscribed in an equilateral triangle with side lengths of \displaystyle 5 as shown by the figure below.

1

Find the area of the shaded region.

Possible Answers:

\displaystyle 10.25

\displaystyle 12.68

\displaystyle 11.03

\displaystyle 10.04

Correct answer:

\displaystyle 10.04

Explanation:

In order to find the area of the shaded region, we must first find the area of the circle and the area of the equilateral triangle.

Recall how to find the area of a circle:

\displaystyle \text{Area of Circle}=\pi\times\text{radius}^2

Plug in the given radius to find the area of the circle.

\displaystyle \text{Area of Circle}=\pi\times( \frac{1}{2})^2=\frac{1}{4}\pi

Next, recall how to find the area of an equilateral triangle:

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(\text{side})^2

Plug in the length of the side of the triangle to find the area.

\displaystyle \text{Area of Equilateral Triangle}=\frac{\sqrt3}{4}(5)^2=\frac{25\sqrt3}{4}

In order to find the area of the shaded region, we will need to subtract the area of the circle from the area of the triangle.

\displaystyle \text{Area of Shaded Region}=\text{Area of Triangle}-\text{Area of Circle}

\displaystyle \text{Area of Shaded Region}=\frac{25\sqrt3}{4}-\frac{1}{4}\pi=10.04

Make sure to round to \displaystyle 2 places after the decimal.

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