Intermediate Geometry : How to find the perimeter of a pentagon

Study concepts, example questions & explanations for Intermediate Geometry

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Example Questions

Example Question #1 : How To Find The Perimeter Of A Pentagon

An apothem is a line drawn from the center of a regular shape to the center of one of its edges. The line drawn is perpendicular to the edge. The apothem of a regular pentagon is . What is the perimeter of this pentagon?

Possible Answers:

Correct answer:

Explanation:

From the given information, we can imagine the following image

Find_the_perimeter_pentagon

In order to solve for the perimeter, we need to solve for the length of one of the sides. This can be accomplished by creating a right triangle from the apothem and the top angle that's marked. 

This angle measure can be calculated through:

, where the sum of all the angles around the center of the pentagon sum up to  and the  is for the number of sides. We can do this because all of the interior angles of the pentagon will be equal because it is regular. Then, this answer needs to be divided by  because the pictured right triangle is half of a larger isosceles triangle. 

Therefore, the marked angle is

Now that one side and one angle are known, we can use SOH CAH TOA because this is a right triangle. In this case, the tangent function will be used because the mystery side of interest is opposite of the known angle, and we are already given the adjacent side (the apothem). In solving for the unknown side, we will label it as .

For a more precise answer, keep the entire number in your calculator. This will prevent rounding errors. 

Keep in mind that the base of the triangle is actually only half the length of the side. This means that the value for  must be multiplied by

In order to solve for the perimeter, the length of the side needs to be multiplied by the number of sides; in this case, there are five sides.

This is the final answer, so the entire decimal is no longer needed. Rounded, the perimeter is .

Example Question #2 : How To Find The Perimeter Of A Pentagon

If a side length of a pentagon is , what is the perimeter of the pentagon?

Possible Answers:

Correct answer:

Explanation:

Write the formula to find the perimeter of a pentagon.

Substitute the side length  into the equation and simplify.

Example Question #3 : How To Find The Perimeter Of A Pentagon

If the side of a pentagon is , what is the perimeter?

Possible Answers:

Correct answer:

Explanation:

Write the perimeter formula for pentagons and substitute the side length.

Example Question #4 : How To Find The Perimeter Of A Pentagon

Determine the perimeter of a pentagon if the side length is .

Possible Answers:

Correct answer:

Explanation:

Write the formula to find the perimeter for a pentagon.

Substitute the side into the perimeter formula.

Example Question #5 : How To Find The Perimeter Of A Pentagon

In the figure below, if the length of is one-third the length of , what is the perimeter of the pentagon?

1

Possible Answers:

Correct answer:

Explanation:

1

Notice that the pentagon is made up of one rectangle and two right triangles. Start by finding the lengths of  and .

Let  be the length of . Then, it follows that  is the length of .

From the figure, we know that . Since we are working with a rectangle, .

Now, solve for .

So then, .

Now that we know , we can use the Pythagorean theorem to solve for the length of .

Plug in the lengths of the legs to find the length of the hypotenuse.

Next, find the length of  by using the Pythagorean theorem.

Finally, add up all the side lengths of the pentagon to find its perimeter.

Make sure to round to  places after the decimal.

 

Example Question #6 : How To Find The Perimeter Of A Pentagon

In pentagon  below, the length of  is one-third that of . Find the perimeter of the pentagon.

5

Possible Answers:

Correct answer:

Explanation:

5

Notice that this pentagon is made up of two right triangles and one rectangle.

Start by finding the length of . Since we are given an angle measurement and a length of a leg of the triangle, we can use tangent to find the length of .

Now, we know that  is three times the length of .

Since we have rectangle .

Thus, 

Now, we can use cosine to find the length of .

Next, use the Pythagorean Theorem to find the length of .

Now that we have all the side lengths of the pentagon, we can find its perimeter.

 

Example Question #7 : How To Find The Perimeter Of A Pentagon

In pentagon  below, the length of  is one-third the length of . Find the perimeter of the pentagon.

6

Possible Answers:

Correct answer:

Explanation:

6

Notice that this pentagon is made up of two right triangles and one rectangle.

Start by finding the length of . Since we are given an angle measurement and a length of a leg of the triangle, we can use tangent to find the length of .

Now, we know that  is three times the length of .

Since we have rectangle .

Thus, 

Now, we can use sine to find the length of .

Next, use the Pythagorean Theorem to find the length of .

Now that we have all the side lengths of the pentagon, we can find its perimeter.

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