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Pre-Algebra : Circumference of a Circle

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Example Questions

Example Question #1 : Circumference Of A Circle

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:

$C=\Pi r^{2}$

$C=l\cdot w$

$C=2\Pi r$

$P=d\Pi$

None of the other answers

Correct answer:

$C=2\Pi r$

Explanation:

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

Correct Answer: $C=2\Pi r$

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Example Question #1 : Circumference Of A Circle

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

Possible Answers:

$18\pi$

$4.5\pi$

$36\pi$

$9\pi$

Correct answer:

$18\pi$

Explanation:

Using the area, solve for the radius:

$A=r^2\pi$

$81\pi=r^2\pi$

Divide by $\pi$ :

81=r^2

Take the square root:

$\sqrt{81}=9=r$

Plug this radius into the formula for the circumference:

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

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Example Question #1 : Circumference Of A Circle

What is the circumference of a circle with a radius equal to ?

Possible Answers:

$7\pi$

$49\pi$

$14\pi$

$196\pi$

Correct answer:

$14\pi$

Explanation:

The circumference can be solved using the following equation:

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

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Example Question #1 : Circumference Of A Circle

What is the circumference of a circle with a radius of ?

Possible Answers:

$16\pi^{2}$

$8\pi$

$28\pi$

$16\pi$

Correct answer:

$16\pi$

Explanation:

The circumference can be solved using the following equation:

$C=2r\pi$

Where represents the radius. Therefore, when we substitute our radius in we get:

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

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Example Question #3 : How To Find Circumference

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

Possible Answers:

$7\pi$ inches

$(30 - 8 \pi )$ inches

$44 \pi$ inches

$22 \pi$ inches

$14 \pi$ inches

Correct answer:

$22 \pi$ inches

Explanation:

The circumference of a circle with radius inches is

$C = 2 \pi r = 2 \pi \cdot 30 = 60 \pi$ inches.

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

2 L + 2W = P

$2 L + 2 \cdot 8 \pi = 60 \pi$

$2 L + 16 \pi = 60 \pi$

$2 L + 16 \pi - 16 \pi = 60 \pi - 16 \pi$

$2 L =44 \pi$

$2 L \div 2=44 \pi \div 2$

$L = 22\pi$ inches

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Example Question #2 : Circumference Of A Circle

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

Possible Answers:

$400\pi$

$200\pi$

$40\pi$

$20\pi$

$80\pi$

Correct answer:

$40\pi$

Explanation:

Write the formula for circumference.

$C=2\pi r$

Substitute the radius and solve.

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

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Example Question #1 : Circumference Of A Circle

Given the area of the circle is $9\pi$ , what is the circumference of the circle?

Possible Answers:

$3+3\pi$

$6\pi +6$

$6\pi$

Correct answer:

$6\pi$

Explanation:

Write the formula for the area of a circle.

$A=\pi r^2$

Substitute the area.

$9\pi=\pi r^2$

r^2=9

r=3

Write the circumference of the circle.

$C=2\pi r$

Substitute the radius.

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

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Example Question #2 : Circumference Of A Circle

Find the circumference of a circle with an area of $\pi^2$ .

Possible Answers:

$3\pi$

$2\pi ^3$

$3\pi^2$

$2\pi \sqrt{\pi}$

$\pi \sqrt{\pi}$

Correct answer:

$2\pi \sqrt{\pi}$

Explanation:

Write the area formula in order to find the radius.

$A=\pi r^2$

Substitute the area.

$\pi ^2=\pi r^2$

$r^2=\pi$

$r=\sqrt{\pi}$

Write the circumference formula.

$C=2\pi r$

Substitute the radius.

$C=2\pi \sqrt{\pi}$

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Example Question #3 : Circumference Of A Circle

Determine the circumference of the circle if the radius is $2\pi$ .

Possible Answers:

$4\pi ^3$

$8\pi$

$2\pi ^2$

$4\pi ^2$

$4\pi$

Correct answer:

$4\pi ^2$

Explanation:

Write the formula for the circumference of a circle.

$C=2\pi r$

Substitute the radius into the formula.

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

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Example Question #4 : Circumference Of A Circle

Find the circumference of a circle with a radius of $\sqrt2$ .

Possible Answers:

$\frac{\sqrt{2\pi}}{2}$

$\frac{\sqrt2}{2} \pi$

$2\pi \sqrt{2}$

$4\pi$

$2\pi$

Correct answer:

$2\pi \sqrt{2}$

Explanation:

Write the formula to find the circumference of a circle.

$C=2\pi r$

Substitute the radius.

$C=2\pi \sqrt{2}$

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Pre-Algebra : Circumference of a Circle

Study concepts, example questions & explanations for Pre-Algebra

All Pre-Algebra Resources

Example Questions

Example Question #1 : Circumference Of A Circle

Example Question #1 : Circumference Of A Circle

Example Question #1 : Circumference Of A Circle

Example Question #1 : Circumference Of A Circle

Example Question #3 : How To Find Circumference

Example Question #2 : Circumference Of A Circle

Example Question #1 : Circumference Of A Circle

Example Question #2 : Circumference Of A Circle

Example Question #3 : Circumference Of A Circle

Example Question #4 : Circumference Of A Circle

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