Pre-Algebra : Perimeter

Study concepts, example questions & explanations for Pre-Algebra

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Example Questions

Example Question #1 : Perimeter

Becky's house is surrounded by a circular lake that Becky runs around each day. If Becky wants to figure out the distance she runs around the lake, what formula should Becky use?

Possible Answers:

\(\displaystyle C=l\cdot w\)

None of the other answers

\(\displaystyle C=2\Pi r\)

\(\displaystyle C=\Pi r^{2}\)

\(\displaystyle P=d\Pi\)

Correct answer:

\(\displaystyle C=2\Pi r\)

Explanation:

 

Becky should use the circumfrence formula to find the distance she runs of a lake.

Correct Answer: \(\displaystyle C=2\Pi r\)

Example Question #2 : Perimeter

What is the circumference of a circle with an area equal to \(\displaystyle 81\pi\)?

Possible Answers:

\(\displaystyle 36\pi\)

\(\displaystyle 4.5\pi\)

\(\displaystyle 18\pi\)

\(\displaystyle 9\pi\)

Correct answer:

\(\displaystyle 18\pi\)

Explanation:

Using the area, solve for the radius:

\(\displaystyle A=r^2\pi\)

\(\displaystyle 81\pi=r^2\pi\)

Divide by \(\displaystyle \pi\):

\(\displaystyle 81=r^2\)

Take the square root:

\(\displaystyle \sqrt{81}=9=r\)

Plug this radius into the formula for the circumference:

\(\displaystyle C=2r\pi=2(9)\pi=18\pi\)

Example Question #1 : Circumference Of A Circle

What is the circumference of a circle with a radius equal to \(\displaystyle 7\)?

Possible Answers:

\(\displaystyle 196\pi\)

\(\displaystyle 7\pi\)

\(\displaystyle 49\pi\)

\(\displaystyle 14\pi\)

Correct answer:

\(\displaystyle 14\pi\)

Explanation:

The circumference can be solved using the following equation:

\(\displaystyle C=2r\pi=2(7)\pi=14\pi\)

Example Question #1 : Circumference Of A Circle

What is the circumference of a circle with a radius of \(\displaystyle 8\)?

Possible Answers:

\(\displaystyle 16\)

\(\displaystyle 16\pi\)

\(\displaystyle 16\pi^{2}\)

\(\displaystyle 28\pi\)

\(\displaystyle 8\pi\)

Correct answer:

\(\displaystyle 16\pi\)

Explanation:

The circumference can be solved using the following equation:

\(\displaystyle C=2r\pi\)

Where \(\displaystyle r\) represents the radius. Therefore, when we substitute our radius in we get:

\(\displaystyle =2(8)\pi =16\pi\)

Example Question #1 : Know And Use The Formulas For The Area And Circumference Of A Circle: Ccss.Math.Content.7.G.B.4

The perimeter of a given rectangle is equal to the circumference of a given circle. The circle has radius \(\displaystyle 30\) inches; the width of the rectangle is \(\displaystyle 8 \pi\) inches. What is the length of the rectangle?

Possible Answers:

\(\displaystyle 7\pi\) inches

\(\displaystyle (30 - 8 \pi )\) inches

\(\displaystyle 14 \pi\) inches

\(\displaystyle 44 \pi\) inches

\(\displaystyle 22 \pi\) inches

Correct answer:

\(\displaystyle 22 \pi\) inches

Explanation:

The circumference of a circle with radius \(\displaystyle 30\) inches is 

\(\displaystyle C = 2 \pi r = 2 \pi \cdot 30 = 60 \pi\) inches.

The perimeter of the rectangle is therefore \(\displaystyle 60 \pi\) inches. To find its length, substitute \(\displaystyle W = 8\pi\) and \(\displaystyle P = 60\pi\) into the equation and solve for \(\displaystyle L\):

\(\displaystyle 2 L + 2W = P\)

\(\displaystyle 2 L + 2 \cdot 8 \pi = 60 \pi\)

\(\displaystyle 2 L + 16 \pi = 60 \pi\)

\(\displaystyle 2 L + 16 \pi - 16 \pi = 60 \pi - 16 \pi\)

\(\displaystyle 2 L =44 \pi\)

\(\displaystyle 2 L \div 2=44 \pi \div 2\)

\(\displaystyle L = 22\pi\) inches

Example Question #3 : Perimeter

Determine the circumference of the circle if the radius is 20.

Possible Answers:

\(\displaystyle 400\pi\)

\(\displaystyle 20\pi\)

\(\displaystyle 200\pi\)

\(\displaystyle 80\pi\)

\(\displaystyle 40\pi\)

Correct answer:

\(\displaystyle 40\pi\)

Explanation:

Write the formula for circumference.

\(\displaystyle C=2\pi r\)

Substitute the radius and solve.

\(\displaystyle C=2\pi(20) = 40\pi\)

Example Question #3 : Circumference Of A Circle

Given the area of the circle is \(\displaystyle 9\pi\), what is the circumference of the circle?

Possible Answers:

\(\displaystyle 3\)

\(\displaystyle 6\pi +6\)

\(\displaystyle 9\)

\(\displaystyle 3+3\pi\)

\(\displaystyle 6\pi\)

Correct answer:

\(\displaystyle 6\pi\)

Explanation:

Write the formula for the area of a circle.

\(\displaystyle A=\pi r^2\)

Substitute the area.

\(\displaystyle 9\pi=\pi r^2\)

\(\displaystyle r^2=9\)

\(\displaystyle r=3\)

Write the circumference of the circle.

\(\displaystyle C=2\pi r\)

Substitute the radius.

\(\displaystyle C=2\pi (3) = 6\pi\)

Example Question #271 : Geometry

Find the circumference of a circle with an area of \(\displaystyle \pi^2\).

Possible Answers:

\(\displaystyle 2\pi ^3\)

\(\displaystyle 2\pi \sqrt{\pi}\) 

\(\displaystyle \pi \sqrt{\pi}\)

\(\displaystyle 3\pi\)

\(\displaystyle 3\pi^2\)

Correct answer:

\(\displaystyle 2\pi \sqrt{\pi}\) 

Explanation:

Write the area formula in order to find the radius.

\(\displaystyle A=\pi r^2\)

Substitute the area.

\(\displaystyle \pi ^2=\pi r^2\)

\(\displaystyle r^2=\pi\)

\(\displaystyle r=\sqrt{\pi}\)

Write the circumference formula.

\(\displaystyle C=2\pi r\)

Substitute the radius.

\(\displaystyle C=2\pi \sqrt{\pi}\)

Example Question #4 : Perimeter

Determine the circumference of the circle if the radius is \(\displaystyle 2\pi\).

Possible Answers:

\(\displaystyle 4\pi ^2\)

\(\displaystyle 8\pi\)

\(\displaystyle 2\pi ^2\)

\(\displaystyle 4\pi ^3\)

\(\displaystyle 4\pi\)

Correct answer:

\(\displaystyle 4\pi ^2\)

Explanation:

Write the formula for the circumference of a circle.

\(\displaystyle C=2\pi r\)

Substitute the radius into the formula.

\(\displaystyle C=2\pi (2\pi) = 4\pi ^2\)

Example Question #5 : Perimeter

Find the circumference of a circle with a radius of \(\displaystyle \sqrt2\).

Possible Answers:

\(\displaystyle \frac{\sqrt2}{2} \pi\)

\(\displaystyle 2\pi\)

\(\displaystyle \frac{\sqrt{2\pi}}{2}\)

\(\displaystyle 4\pi\)

\(\displaystyle 2\pi \sqrt{2}\)

Correct answer:

\(\displaystyle 2\pi \sqrt{2}\)

Explanation:

Write the formula to find the circumference of a circle.

\(\displaystyle C=2\pi r\)

Substitute the radius.

\(\displaystyle C=2\pi \sqrt{2}\)

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