SSAT Upper Level Math : How to find the area of a pentagon

Study concepts, example questions & explanations for SSAT Upper Level Math

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Example Questions

Example Question #754 : Geometry

Find the exact area of a pentagon if the side length is \(\displaystyle 6\).

Possible Answers:

\(\displaystyle \sqrt{25+10\sqrt5}\)

\(\displaystyle 9\sqrt{10\sqrt5}\)

\(\displaystyle 15+90\sqrt5\)

\(\displaystyle 6\sqrt{35\sqrt5}\)

\(\displaystyle 9\sqrt{25+10\sqrt5}\)

Correct answer:

\(\displaystyle 9\sqrt{25+10\sqrt5}\)

Explanation:

Write the formula for the area of a pentagon.

\(\displaystyle A=\frac{s^{2}}{4}\sqrt{5(5+2\sqrt5)}\)

Substitute the side length and simplify.

\(\displaystyle A=\frac{6^{2}}{4}\sqrt{5(5+2\sqrt5)}\)

\(\displaystyle A= 9\sqrt{25+10\sqrt5}\)

Example Question #755 : Geometry

In terms of \(\displaystyle x\), find the area of a regular pentagon that has a side length of \(\displaystyle 6x\) and an apothem of \(\displaystyle 2x\).

Possible Answers:

\(\displaystyle 30x^2\)

\(\displaystyle 30x\)

\(\displaystyle 60x^2\)

\(\displaystyle 120x^2\)

Correct answer:

\(\displaystyle 30x^2\)

Explanation:

To find the area of a regular polygon,

\(\displaystyle \text{Area}=\frac{1}{2}(Perimeter)(apothem)\)

To find the perimeter of the pentagon,

\(\displaystyle \text{Perimeter}=5(side)\)

For the given pentagon,

\(\displaystyle \text{Perimeter}=5\times 6x=30x\)

So then, to find the area of the pentagon,

\(\displaystyle \text{Area}=\frac{1}{2}(30x)(2x)=30x^2\)

Example Question #756 : Geometry

Find the area of a regular pentagon that has a side length of \(\displaystyle 20\) and an apothem of \(\displaystyle 15\).

Possible Answers:

\(\displaystyle 1500\)

\(\displaystyle 500\)

\(\displaystyle 2000\)

\(\displaystyle 750\)

Correct answer:

\(\displaystyle 750\)

Explanation:

To find the area of a regular polygon,

\(\displaystyle \text{Area}=\frac{1}{2}(Perimeter)(apothem)\)

To find the perimeter of the pentagon,

\(\displaystyle \text{Perimeter}=5(side)\)

For the given pentagon,

\(\displaystyle \text{Perimeter}=5\times20=100\)

So then, to find the area of the pentagon,

\(\displaystyle \text{Area}=\frac{1}{2}(100)(15)=750\)

Example Question #21 : Areas And Perimeters Of Polygons

Find the area of a regular pentagon with a side length of \(\displaystyle 4\).

Possible Answers:

\(\displaystyle 41.21\)

\(\displaystyle 98.21\)

\(\displaystyle 29.11\)

\(\displaystyle 27.53\)

Correct answer:

\(\displaystyle 27.53\)

Explanation:

Use the following formula to find the area of a regular pentagon:

\(\displaystyle \text{Area}=\frac{1}{4}\sqrt{5(5+2\sqrt5)}(side)^2\)

Plugging in the information given by the question,

\(\displaystyle \text{Area}=\frac{1}{4}\sqrt{5(5+2\sqrt5)}(4)^2=27.53\)

Example Question #758 : Geometry

In terms of \(\displaystyle z\), find the area of a regular pentagon with side lengths of \(\displaystyle 5z\) and an apothem of \(\displaystyle 6\).

Possible Answers:

\(\displaystyle 75z\)

\(\displaystyle 50z\)

\(\displaystyle 150z\)

\(\displaystyle 75z^2\)

Correct answer:

\(\displaystyle 75z\)

Explanation:

To find the area of a regular polygon,

\(\displaystyle \text{Area}=\frac{1}{2}(Perimeter)(apothem)\)

To find the perimeter of the pentagon,

\(\displaystyle \text{Perimeter}=5(side)\)

For the given pentagon,

\(\displaystyle \text{Perimeter}=5\times 5z =25z\)

So then, to find the area of the pentagon,

\(\displaystyle \text{Area}=\frac{1}{2}(25z)(6)=75z\)

Example Question #761 : Geometry

In terms of \(\displaystyle a\), find the area of a regular pentagon that has a side length of \(\displaystyle 8a\) and an apothem of \(\displaystyle 4a\).

Possible Answers:

\(\displaystyle 160a^2\)

\(\displaystyle 120a^2\)

\(\displaystyle 80a\)

\(\displaystyle 80a^2\)

Correct answer:

\(\displaystyle 80a^2\)

Explanation:

To find the area of a regular polygon,

\(\displaystyle \text{Area}=\frac{1}{2}(Perimeter)(apothem)\)

To find the perimeter of the pentagon,

\(\displaystyle \text{Perimeter}=5(side)\)

For the given pentagon,

\(\displaystyle \text{Perimeter}=5\times 8a=40a\)

So then, to find the area of the pentagon,

\(\displaystyle \text{Area}=\frac{1}{2}(40a)(4a)=80a^2\)

Example Question #762 : Geometry

In terms of \(\displaystyle b\), find the area of a regular pentagon with side lengths of \(\displaystyle 12b\) and an apothem of \(\displaystyle 10b\).

Possible Answers:

\(\displaystyle 200b^2\)

\(\displaystyle 300b^2\)

\(\displaystyle 600b^2\)

\(\displaystyle 400b^2\)

Correct answer:

\(\displaystyle 300b^2\)

Explanation:

To find the area of a regular polygon,

\(\displaystyle \text{Area}=\frac{1}{2}(Perimeter)(apothem)\)

To find the perimeter of the pentagon,

\(\displaystyle \text{Perimeter}=5(side)\)

For the given pentagon,

\(\displaystyle \text{Perimeter}=5\times 12b=60b\)

So then, to find the area of the pentagon,

\(\displaystyle \text{Area}=\frac{1}{2}(60b)(10b)=300b^2\)

Example Question #763 : Geometry

The area of a regular polygon is \(\displaystyle 512\). If the length of its apothem is \(\displaystyle 8\), what is the length of each side of the pentagon?

Possible Answers:

\(\displaystyle 22.2\)

\(\displaystyle 18.2\)

\(\displaystyle 25.6\)

\(\displaystyle 20.8\)

Correct answer:

\(\displaystyle 25.6\)

Explanation:

To find the area of a regular polygon,

\(\displaystyle \text{Area}=\frac{1}{2}(Perimeter)(apothem)\)

For the given pentagon,

\(\displaystyle 512=\frac{1}{2}(Perimeter)(8)\)

\(\displaystyle 8(Perimeter)=1024\)

\(\displaystyle Perimeter=128\)

To find the length of each side of the pentagon, divide the perimeter by \(\displaystyle 5\).

\(\displaystyle Side=25.6\)

Example Question #21 : Area Of Polygons

A regular pentagon has an area of \(\displaystyle 250\). Find the length of each side of the pentagon if the apothem has a length of \(\displaystyle 5\).

Possible Answers:

\(\displaystyle 10\)

\(\displaystyle 15\)

\(\displaystyle 20\)

\(\displaystyle 25\)

Correct answer:

\(\displaystyle 20\)

Explanation:

To find the area of a regular polygon,

\(\displaystyle \text{Area}=\frac{1}{2}(Perimeter)(apothem)\)

For the given pentagon,

\(\displaystyle 250=\frac{1}{2}(Perimeter)(5)\)

\(\displaystyle 5(Perimeter)=500\)

\(\displaystyle Perimeter=100\)

To find the length of each side of the pentagon, divide the perimeter by \(\displaystyle 5\).

\(\displaystyle Side=20\)

Example Question #22 : Area Of Polygons

The area of a regular pentagon is \(\displaystyle 410\). Find the length of a side of the pentagon if the apothem has a length of \(\displaystyle 12\).

Possible Answers:

\(\displaystyle \frac{41}{3}\)

\(\displaystyle \frac{61}{3}\)

\(\displaystyle 20\)

\(\displaystyle \frac{15}{2}\)

Correct answer:

\(\displaystyle \frac{41}{3}\)

Explanation:

To find the area of a regular polygon,

\(\displaystyle \text{Area}=\frac{1}{2}(Perimeter)(apothem)\)

For the given pentagon,

\(\displaystyle 410=\frac{1}{2}(Perimeter)(12)\)

\(\displaystyle 12(Perimeter)=820\)

\(\displaystyle Perimeter=\frac{820}{12}\)

To find the length of each side of the pentagon, divide the perimeter by \(\displaystyle 5\).

\(\displaystyle Side=\frac{820}{60}=\frac{41}{3}\)

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