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

Study concepts, example questions & explanations for Intermediate Geometry

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Example Questions

Example Question #1 : How To Find The Perimeter Of An Equilateral Triangle

ΔABC is an equilateral triangle with area 55. 

Find the perimeter (to the nearest tenth).

Possible Answers:

33.8

37.2

40.3

35.9

39.1

Correct answer:

33.8

Explanation:

To find the perimeter of an equilateral triangle given its area, we must first find the length of the sides. This can be done by using the equation of the area of an equilateral triangle:

\(\displaystyle Area =\frac{a^2\sqrt{3}}{4}\)

where a is the side of the triangle.

Because the sides of the equilateral triangle are equal, the perimeter is equal to 3a.

Example Question #1 : How To Find The Perimeter Of An Equilateral Triangle

ΔABC is an equilateral triangle with area 28. 

Find the perimeter (to the nearest tenth).

Possible Answers:

20.8

24.1

26.7

19.3

25.8

Correct answer:

24.1

Explanation:

To find the perimeter of an equilateral triangle given its area, we must first find the length of the sides. This can be done by using the equation of the area of an equilateral triangle:

\(\displaystyle Area =\frac{a^2\sqrt{3}}{4}\)

where a is the side of the triangle.

Because the sides of the equilateral triangle are equal, the perimeter is equal to 3a.

Example Question #2 : How To Find The Perimeter Of An Equilateral Triangle

An equilateral triangle has a side length of \(\displaystyle 10\) cm.

What is the perimeter of the triangle?

Possible Answers:

\(\displaystyle 1000cm\)

\(\displaystyle 25cm\)

\(\displaystyle 50cm\)

\(\displaystyle 30cm\)

Correct answer:

\(\displaystyle 30cm\)

Explanation:

The perimeter of a triangle is the sum of all three sides.

Because an equilateral triangle has all three sides of equal length, we have

\(\displaystyle P = 10 +10+10=30cm\)

Example Question #81 : Equilateral Triangles

An equilateral triangle has a side length of \(\displaystyle 5\) cm.

What is the perimeter of the triangle?

Possible Answers:

\(\displaystyle 10cm\)

\(\displaystyle 15cm\)

\(\displaystyle 20cm\)

\(\displaystyle 125cm\)

Correct answer:

\(\displaystyle 15cm\)

Explanation:

The perimeter of a triangle is the sum of all three sides.

Because an equilateral triangle has all three sides of equal length, we have

\(\displaystyle P = 5+5+5=15cm\)

Example Question #5 : How To Find The Perimeter Of An Equilateral Triangle

An equilateral triangle is placed together with a semicircle as shown by the figure below.

1

Find the perimeter of the figure.

Possible Answers:

\(\displaystyle 10.28\)

\(\displaystyle 14.68\)

\(\displaystyle 12.37\)

\(\displaystyle 13.22\)

Correct answer:

\(\displaystyle 12.37\)

Explanation:

13

In order to find the perimeter of the entire figure, we will need to find the lengths of the segments highlighted in red.

Notice that the side length of the equilateral triangle is equal to the diameter of the semicircle.

Next, you should recall that the height of an equilateral triangle splits the triangle into two congruent \(\displaystyle 30-60-90\) triangles.

Recall that the side lengths in a \(\displaystyle 30-60-90\) triangle are in a \(\displaystyle 1:\sqrt3:2\) ratio. Thus, the radius of the circle, which is also the base of the \(\displaystyle 30-60-90\) triangle, the height of the triangle, and the side length of the triangle are in the same ratio.

We can then set up the following to determine the length of the side of the equilateral triangle:

\(\displaystyle \frac{\text{height}}{\sqrt3}=\frac{\text{side}}{2}\)

Rearrange the equation to solve for the length of the side.

\(\displaystyle \text{side}=\frac{2(\text{height})}{\sqrt3}=\frac{2\sqrt3(\text{height})}{3}\)

Plug in the length of the height to find the length of the side.

\(\displaystyle \text{side}=\frac{2\sqrt3(3)}{3}=2\sqrt3\)

Since the diameter of the semi-circle and the length of a side of the equilateral triangle are the same, we can write the following equation:

\(\displaystyle \text{side}=\text{diameter}\)

We have two sides of the equilateral triangle and the circumference of a semi-circle.

\(\displaystyle \text{Perimeter}=2(\text{side})+\frac{\pi(\text{side})}{2}\)

Plug in the length of the side to find the perimeter.

\(\displaystyle \text{Perimeter}=2(2\sqrt3)+\frac{\pi(2\sqrt3)}{2}=4\sqrt3+\pi\sqrt3=12.37\)

Example Question #6 : How To Find The Perimeter Of An Equilateral Triangle

An equilateral triangle is placed together with a semicircle as shown by the figure below.

2

Find the perimeter of the figure.

Possible Answers:

\(\displaystyle 24.69\)

\(\displaystyle 22.38\)

\(\displaystyle 25.09\)

\(\displaystyle 24.74\)

Correct answer:

\(\displaystyle 24.74\)

Explanation:

13

In order to find the perimeter of the entire figure, we will need to find the lengths of the segments highlighted in red.

Notice that the side length of the equilateral triangle is equal to the diameter of the semicircle.

Next, you should recall that the height of an equilateral triangle splits the triangle into two congruent \(\displaystyle 30-60-90\) triangles.

Recall that the side lengths in a \(\displaystyle 30-60-90\) triangle are in a \(\displaystyle 1:\sqrt3:2\) ratio. Thus, the radius of the circle, which is also the base of the \(\displaystyle 30-60-90\) triangle, the height of the triangle, and the side length of the triangle are in the same ratio.

We can then set up the following to determine the length of the side of the equilateral triangle:

\(\displaystyle \frac{\text{height}}{\sqrt3}=\frac{\text{side}}{2}\)

Rearrange the equation to solve for the length of the side.

\(\displaystyle \text{side}=\frac{2(\text{height})}{\sqrt3}=\frac{2\sqrt3(\text{height})}{3}\)

Plug in the length of the height to find the length of the side.

\(\displaystyle \text{side}=\frac{2\sqrt3(6)}{3}=4\sqrt3\)

Since the diameter of the semi-circle and the length of a side of the equilateral triangle are the same, we can write the following equation:

\(\displaystyle \text{side}=\text{diameter}\)

We have two sides of the equilateral triangle and the circumference of a semi-circle.

\(\displaystyle \text{Perimeter}=2(\text{side})+\frac{\pi(\text{side})}{2}\)

Plug in the length of the side to find the perimeter.

\(\displaystyle \text{Perimeter}=2(4\sqrt3)+\frac{\pi(4\sqrt3)}{2}=8\sqrt3+2\pi\sqrt3=24.74\)

Example Question #7 : How To Find The Perimeter Of An Equilateral Triangle

An equilateral triangle is placed together with a semicircle as shown by the figure below.

3

Find the perimeter of the figure.

Possible Answers:

\(\displaystyle 24.09\)

\(\displaystyle 28.86\)

\(\displaystyle 31.59\)

\(\displaystyle 30.60\)

Correct answer:

\(\displaystyle 28.86\)

Explanation:

13

In order to find the perimeter of the entire figure, we will need to find the lengths of the segments highlighted in red.

Notice that the side length of the equilateral triangle is equal to the diameter of the semicircle.

Next, you should recall that the height of an equilateral triangle splits the triangle into two congruent \(\displaystyle 30-60-90\) triangles.

Recall that the side lengths in a \(\displaystyle 30-60-90\) triangle are in a \(\displaystyle 1:\sqrt3:2\) ratio. Thus, the radius of the circle, which is also the base of the \(\displaystyle 30-60-90\) triangle, the height of the triangle, and the side length of the triangle are in the same ratio.

We can then set up the following to determine the length of the side of the equilateral triangle:

\(\displaystyle \frac{\text{height}}{\sqrt3}=\frac{\text{side}}{2}\)

Rearrange the equation to solve for the length of the side.

\(\displaystyle \text{side}=\frac{2(\text{height})}{\sqrt3}=\frac{2\sqrt3(\text{height})}{3}\)

Plug in the length of the height to find the length of the side.

\(\displaystyle \text{side}=\frac{2\sqrt3(7)}{3}=\frac{14\sqrt3}{3}\)

Since the diameter of the semi-circle and the length of a side of the equilateral triangle are the same, we can write the following equation:

\(\displaystyle \text{side}=\text{diameter}\)

We have two sides of the equilateral triangle and the circumference of a semi-circle.

\(\displaystyle \text{Perimeter}=2(\text{side})+\frac{\pi(\text{side})}{2}\)

Plug in the length of the side to find the perimeter.

\(\displaystyle \text{Perimeter}=2(\frac{14\sqrt3}{3})+\frac{\pi(\frac{14\sqrt3}{3})}{2}=\frac{28\sqrt3}{3}+\frac{7\pi\sqrt3}{3}=28.86\)

Example Question #8 : How To Find The Perimeter Of An Equilateral Triangle

An equilateral triangle is placed together with a semicircle as shown by the figure below.

4

Find the perimeter of the figure.

Possible Answers:

\(\displaystyle 41.28\)

\(\displaystyle 45.36\)

\(\displaystyle 47.09\)

\(\displaystyle 40.09\)

Correct answer:

\(\displaystyle 45.36\)

Explanation:

13

In order to find the perimeter of the entire figure, we will need to find the lengths of the segments highlighted in red.

Notice that the side length of the equilateral triangle is equal to the diameter of the semicircle.

Next, you should recall that the height of an equilateral triangle splits the triangle into two congruent \(\displaystyle 30-60-90\) triangles.

Recall that the side lengths in a \(\displaystyle 30-60-90\) triangle are in a \(\displaystyle 1:\sqrt3:2\) ratio. Thus, the radius of the circle, which is also the base of the \(\displaystyle 30-60-90\) triangle, the height of the triangle, and the side length of the triangle are in the same ratio.

We can then set up the following to determine the length of the side of the equilateral triangle:

\(\displaystyle \frac{\text{height}}{\sqrt3}=\frac{\text{side}}{2}\)

Rearrange the equation to solve for the length of the side.

\(\displaystyle \text{side}=\frac{2(\text{height})}{\sqrt3}=\frac{2\sqrt3(\text{height})}{3}\)

Plug in the length of the height to find the length of the side.

\(\displaystyle \text{side}=\frac{2\sqrt3(11)}{3}=\frac{22\sqrt3}{3}\)

Since the diameter of the semi-circle and the length of a side of the equilateral triangle are the same, we can write the following equation:

\(\displaystyle \text{side}=\text{diameter}\)

We have two sides of the equilateral triangle and the circumference of a semi-circle.

\(\displaystyle \text{Perimeter}=2(\text{side})+\frac{\pi(\text{side})}{2}\)

Plug in the length of the side to find the perimeter.

\(\displaystyle \text{Perimeter}=2(\frac{22\sqrt3}{3})+\frac{\pi(\frac{22\sqrt3}{3})}{2}=\frac{44\sqrt3}{3}+\frac{11\pi\sqrt3}{3}=45.36\)

Example Question #9 : How To Find The Perimeter Of An Equilateral Triangle

An equilateral triangle is placed together with a semicircle as shown in the figure below.

5

Find the perimeter of the figure.

Possible Answers:

\(\displaystyle 56.17\)

\(\displaystyle 53.60\)

\(\displaystyle 51.24\)

\(\displaystyle 55.82\)

Correct answer:

\(\displaystyle 53.60\)

Explanation:

13

In order to find the perimeter of the entire figure, we will need to find the lengths of the segments highlighted in red.

Notice that the side length of the equilateral triangle is equal to the diameter of the semicircle.

Next, you should recall that the height of an equilateral triangle splits the triangle into two congruent \(\displaystyle 30-60-90\) triangles.

Recall that the side lengths in a \(\displaystyle 30-60-90\) triangle are in a \(\displaystyle 1:\sqrt3:2\) ratio. Thus, the radius of the circle, which is also the base of the \(\displaystyle 30-60-90\) triangle, the height of the triangle, and the side length of the triangle are in the same ratio.

We can then set up the following to determine the length of the side of the equilateral triangle:

\(\displaystyle \frac{\text{height}}{\sqrt3}=\frac{\text{side}}{2}\)

Rearrange the equation to solve for the length of the side.

\(\displaystyle \text{side}=\frac{2(\text{height})}{\sqrt3}=\frac{2\sqrt3(\text{height})}{3}\)

Plug in the length of the height to find the length of the side.

\(\displaystyle \text{side}=\frac{2\sqrt3(13)}{3}=\frac{26\sqrt3}{3}\)

Since the diameter of the semi-circle and the length of a side of the equilateral triangle are the same, we can write the following equation:

\(\displaystyle \text{side}=\text{diameter}\)

We have two sides of the equilateral triangle and the circumference of a semi-circle.

\(\displaystyle \text{Perimeter}=2(\text{side})+\frac{\pi(\text{side})}{2}\)

Plug in the length of the side to find the perimeter.

\(\displaystyle \text{Perimeter}=2(\frac{26\sqrt3}{3})+\frac{\pi(\frac{26\sqrt3}{3})}{2}=\frac{52\sqrt3}{3}+\frac{13\pi\sqrt3}{3}=53.60\)

Example Question #10 : How To Find The Perimeter Of An Equilateral Triangle

An equilateral triangle is placed together with a semicircle as shown by the figure below.

6

Find the perimeter of the figure.

Possible Answers:

\(\displaystyle 62.81\)

\(\displaystyle 70.09\)

\(\displaystyle 71.91\)

\(\displaystyle 65.34\)

Correct answer:

\(\displaystyle 70.09\)

Explanation:

13

In order to find the perimeter of the entire figure, we will need to find the lengths of the segments highlighted in red.

Notice that the side length of the equilateral triangle is equal to the diameter of the semicircle.

Next, you should recall that the height of an equilateral triangle splits the triangle into two congruent \(\displaystyle 30-60-90\) triangles.

Recall that the side lengths in a \(\displaystyle 30-60-90\) triangle are in a \(\displaystyle 1:\sqrt3:2\) ratio. Thus, the radius of the circle, which is also the base of the \(\displaystyle 30-60-90\) triangle, the height of the triangle, and the side length of the triangle are in the same ratio.

We can then set up the following to determine the length of the side of the equilateral triangle:

\(\displaystyle \frac{\text{height}}{\sqrt3}=\frac{\text{side}}{2}\)

Rearrange the equation to solve for the length of the side.

\(\displaystyle \text{side}=\frac{2(\text{height})}{\sqrt3}=\frac{2\sqrt3(\text{height})}{3}\)

Plug in the length of the height to find the length of the side.

\(\displaystyle \text{side}=\frac{2\sqrt3(17)}{3}=\frac{34\sqrt3}{3}\)

Since the diameter of the semi-circle and the length of a side of the equilateral triangle are the same, we can write the following equation:

\(\displaystyle \text{side}=\text{diameter}\)

We have two sides of the equilateral triangle and the circumference of a semi-circle.

\(\displaystyle \text{Perimeter}=2(\text{side})+\frac{\pi(\text{side})}{2}\)

Plug in the length of the side to find the perimeter.

\(\displaystyle \text{Perimeter}=2(\frac{34\sqrt3}{3})+\frac{\pi(\frac{34\sqrt3}{3})}{2}=\frac{68\sqrt3}{3}+\frac{17\pi\sqrt3}{3}=70.09\)

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