How to find circumference - GRE Quantitative Reasoning

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Question

What is the circumference of a circle with an area of 36π?

Answer

We know that the area of a circle can be expressed: a = πr2

If we know that the area is 36π, we can substitute this into said equation and get: 36π = πr2

Solving for r, we get: 36 = r2; (after taking the square root of both sides:) 6 = r

Now, we know that the circuference of a circle is expressed: c = πd. Since we know that d = 2r (two radii, placed one after the other, make a diameter), we can rewrite the circumference equation to be: c = 2πr

Since we have r, we can rewrite this to be: c = 2π*6 = 12π

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Question

Circle A has an area of $121\pi$ . What is the perimeter of an enclosed semi-circle with half the radius of circle A?

Answer

Based on our information, we know that the 121π = πr2; 121 = r2; r = 11.

Our other circle with half the radius of A has a diameter equal to the radius of A. Therefore, the circumference of this circle is 11π. Half of this is 5.5π. However, since this is a semi circle, it is enclosed and looks like this:

Therefore, we have to include the diameter in the perimeter. Therefore, the total perimeter of the semi-circle is 5.5π + 11.

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Question

Which is greater: the circumference of a circle with an area of $25\pi \ in^2$ , or the perimeter of a square with side length inches?

Answer

Starting with the circle, we need to find the radius in order to get the circumference. Find $\dpi{100} \small r$ by plugging our given area into the equation for the area of a circle:

$A = \pi r^2$

$25\pi = \pi r^2$

$25 = r^2$

$r = 5\ in$

Then calculate circumference:

$C = 2\pi r$

$\dpi{100} \small C = 2\pi \times 5 = 10\pi \approx 31.4 inches$ (approximating $\dpi{100} \small \pi$ as 3.14)

To find the perimeter of the square, we can use $P = 4s$ , where $\dpi{100} \small P$ is the perimeter and $\dpi{100} \small s$ is the side length:

$P = 4 \times 7\ in = 28\ in$

$\dpi{100} \small 31.4>28$ , so the circle's circumference is greater.

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Question

$x\ge 1$

Quantity A: The circumference of a circle with radius

Quantity B: The area of a circle with a diameter one fourth the radius of the circle in Quantity A

Which of the following is true?

Answer

Let's compute each value separately. We know that the radii are positive numbers that are greater than or equal to . This means that we do not need to worry about the fact that the area could represent a square of a decimal value like .

Quantity A

Since $c=2\pi r$ , we know:

$a=2 \cdot \pi \cdot 24x = 48x\pi$

Quantity B

If the diameter is one-fourth the radius of A, we know:
$d = \frac{1}{4}\cdot 24x = 6x$

Thus, the radius must be half of that, or .

Now, we need to compute the area of this circle. We know:

$A = \pi r^2$

Therefore, $A = \pi \cdot (3x)^2 = 9x^2$

Now, notice that if , Quantity A is larger.

However, if we choose a value like , we have:

Quantity A: $48 * 1000 \pi = 48000\pi$

Quantity B: $9 * 1000^2\pi= 9000000\pi$

Therefore, the relation cannot be determined!

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Question

Circle has a center in the center of Square .

The area of Square is .

What is the circumference of Circle ?

Answer

Since we know that the area of Square is , we know , where is the length of one of its sides. From this, we can solve for by taking the square root of both sides. You will have to do this by estimating upward. Therefore, you know that is . By careful guessing, you can quickly see that is . From this, you know that the diameter of your circle must be half of , or (because it is circumscribed). Therefore, you can draw:

The circumference of this circle is defined as:

$C=2\pi r$ or, for your values:

$c=2\pi * 12 = 24\pi$

(You could also compute this from the diameter, but many students just memorize the formula above.)

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