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PSAT Math : Circles

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Example Questions

Example Question #11 : Radius

Possible Answers:

50π

10π

20π

100π

25π

Correct answer:

50π

Explanation:

Circle_graph_area2

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Example Question #71 : Plane Geometry

A circle with diameter of length $\dpi{100} d$ is inscribed in a square. Which of the following is equivalent to the area inside of the square, but outside of the circle?

Possible Answers:

$(\pi - 1)d^2$

$\pi d^{2}+2$

$d^{2}-4$

$\frac{(\pi - 1)d^2}{4}$

$\frac{(4-\pi)d^2}{4}$

Correct answer:

$\frac{(4-\pi)d^2}{4}$

Explanation:

In order to find the area that is inside the square but outside the circle, we will need to subtract the area of the circle from the area of the square. The area of a circle is equal to $\pi r^2$ . However, since we are given the length of the diameter, we will need to solve for the radius in terms of the diameter. Because the diameter of a circle is twice the length of its radius, we can write the following equation and solve for $\dpi{100} r$ :

$\dpi{100} d=2r$

Divide both sides by 2.

$r = \frac{d}{2}$

We will now substitute this into the formula for the area of the circle.

area of circle = $\pi r^2 = \pi(\frac{d}{2})^2=\pi \frac{d}{2}\cdot \frac{d}{2}= \frac{\pi d^2}{4}$

We next will need to find the area of the square. Because the circle is inscribed in the square, the diameter of the circle is equal to the length of the circle's side. In other words, the square has side lengths equal to d. The area of any square is equal to the square of its side length. Therefore, the area of the square is $d^{2}$ .

area of square = $d^{2}$

Lastly, we will subtract the area of the circle from the area of the square.

difference in areas = $d^2 - \frac{\pi d^2}{4}$

We will rewrite $d^{2}$ so that its denominator is 4.

difference in areas = $\frac{4d^2}{4}-\frac{\pi d^2}{4}=\frac{(4-\pi)d^2}{4}$

The answer is $\frac{(4-\pi)d^2}{4}$ .

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Example Question #84 : Plane Geometry

An original circle has an area of $16\pi$ . If the radius is increased by a factor of 3, what is the ratio of the new area to the old area?

Possible Answers:

$7:1$

$3:1$

$10:1$

$9:1$

$8:1$

Correct answer:

$9:1$

Explanation:

The formula for the area of a circle is $\pi r^{2}$ . If we increase $r$ by a factor of 3, we will increase the area by a factor of 9.

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Example Question #13 : Radius

A square has an area of $36\ in^{2}$ . If the side of the the square is the same as the diameter of a circle, what is the area of the circle?

Possible Answers:

$36\pi \ in^{2}$

$18\pi \ in^{2}$

$27\pi \ in^{2}$

$12\pi \ in^{2}$

$9\pi \ in^{2}$

Correct answer:

$9\pi \ in^{2}$

Explanation:

The area of a square is given by $A = s^{2}$ so we know that the side of the square is 6 in. If a circle has a diameter of 6 in, then the radius is 3 in. So the area of the circle is $A = \pi r^{2}$ or $9\pi \ in^{2}$ .

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Example Question #21 : Radius

Mary has a decorative plate with a diameter of ten inches. She places the plate on a rectangular placemat with a length of 18 inches and a width of 12 inches. How much of the placemat is visible?

Possible Answers:

$216\hspace{1 mm}inches^2$

$216\pi\hspace{1 mm}inches^2$

$25\pi\hspace{1 mm}inches^2$

$191\pi\hspace{1 mm}inches^2$

$216-25\pi\hspace{1 mm}inches^2$

Correct answer:

$216-25\pi\hspace{1 mm}inches^2$

Explanation:

First we will calculate the total area of the placemat:

$A=l\times w= 18\times 12= 216\hspace{1 mm}inches^2$

Next we will calculate the area of the circular place

$A=\pi r^2$

And

$d=2r=10$

$r=5\hspace{1 mm}inches$

$A=\pi r^2=\pi (5^2)=25\pi\hspace{1 mm}inches^2$

We will subtract the area of the plate from the total area

$216-25\pi\hspace{1 mm}inches^2$

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Example Question #22 : Radius

The picture above contains both a circle with diameter 4, and a rectangle with length 8 and width 5. Find the area of the shaded region. Round your answer to the nearest integer

Possible Answers:

Correct answer:

Explanation:

First, recall that the diameter of a circle is twice the value of the radius. Therefore a circle with diameter 4 has a radius of 2. Next recall that the area of a circle with radius r=2 is:

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

The area of the rectangle is the length times the width:

$L \times W = 8 \times 5 = 40$

The area of the shaded region is the difference between the 2 areas:

$40 - 4 \pi \approx 40-4(3.14) = 27.44$

The nearest integer is 27.

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Example Question #72 : Plane Geometry

Allen was running around the park when he lost his keys. He was running around aimlessly for the past 30 minutes. When he checked 10 minutes ago, he still had his keys. Allen guesses that he has been running at about 3m/s.

If Allen can check 1 square kilometer per hour, what is the longest it will take him to find his keys?

Possible Answers:

$\sim 100\ hours$

$\sim 81\ hours$

$\sim 10\ hours$

$\sim 1,000\ hours$

$\sim 3\ hours$

Correct answer:

$\sim 10\ hours$

Explanation:

Allen has been running for 10 minutes since he lost his keys at 3m/s. This gives us a maximum distance of 3*600=1800m from his current location. If we move 1800m in all directions, this gives us a circle with radius of 1800m. The area of this circle is

$1800^2\pi \approx 10,178,760m^2$

Our answer, however, is asked for in kilometers. 1800m=1.8km, so our actual area will be $\frac{10,178,760}{1,000,000} = 10.2$ square kilometers. Since he can search 1 per hour, it will take him at most 10.2 hours to find his keys.

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Example Question #41 : How To Find The Area Of A Circle

A 12x16 rectangle is inscribed in a circle. What is the area of the circle?

Possible Answers:

50π

10π

120π

100π

90π

Correct answer:

100π

Explanation:

Explanation: Visualizing the rectangle inside the circle (corners touching the circumference of the circle and the center of the rectangle is the center of the circle) you will see that the rectangle can be divided into 8 congruent right triangles, with the hypotenuse as the radius of the circle. Calculating the radius you divide each side of the rectangle by two for the sides of each right triangle (giving 6 and 8). The hypotenuse (by pythagorean theorem or just knowing right triangle sets) the hypotenuse is give as 10. Area of a circle is given by πr². 10² is 100, so 100π is the area.

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Example Question #42 : How To Find The Area Of A Circle

A circle is inscribed in a square whose side is 6 in. What is the difference in area between the square and the circle, rounded to the nearest square inch?

Possible Answers:

$14\ in^{2}$

$12\ in^{2}$

$4\ in^{2}$

$11\ in^{2}$

$8\ in^{2}$

Correct answer:

$8\ in^{2}$

Explanation:

The circle is inscribed in a square when it is drawn within the square so as to touch in as many places as possible. This means that the side of the square is the same as the diameter of the circle.

Let $\pi =3.14$

$A_{square}= s^{2} = (6)^{2} = 36 in^{2}$

$A_{circle}= \pi r^{2}=3.14\cdot3^{2}=3.14\cdot9=28.26 in^{2}$

So the approximate difference is in area $8\ \textup{in}^{2}$

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Example Question #231 : High School Math

Two equal circles are cut out of a rectangular sheet of paper with the dimensions 10 by 20. The circles were made to have the greatest possible diameter. What is the approximate area of the paper after the two circles have been cut out?

Figure_2

Possible Answers:

Correct answer:

Explanation:

The length of 20 represents the diameters of both circles. Each circle has a diameter of 10 and since radius is half of the diameter, each circle has a radius of 5. The area of a circle is A = πr² . The area of one circle is 25π. The area of both circles is 50π. The area of the rectangle is (10)(20) = 200. 200 - 50π gives you the area of the paper after the two circles have been cut out. π is about 3.14, so 200 – 50(3.14) = 43.

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PSAT Math : Circles

Study concepts, example questions & explanations for PSAT Math

All PSAT Math Resources

Example Questions

Example Question #11 : Radius

Example Question #71 : Plane Geometry

Example Question #84 : Plane Geometry

Example Question #13 : Radius

Example Question #21 : Radius

Example Question #22 : Radius

Example Question #72 : Plane Geometry

Example Question #41 : How To Find The Area Of A Circle

Example Question #42 : How To Find The Area Of A Circle

Example Question #231 : High School Math

All PSAT Math Resources

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