Radius

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PSAT Math › Radius

Questions 1 - 10
1

Circle_graph_area3

100_π_

50_π_

25_π_

10_π_

20_π_

Explanation

Circle_graph_area2

2

A square with a side length of 4 inches is inscribed in a circle, as shown below. What is the area of the unshaded region inside of the circle, in square inches?

Act_math_01

8π - 16

4π-4

8π-4

2π-4

8π-8

Explanation

Using the Pythagorean Theorem, the diameter of the circle (also the diagonal of the square) can be found to be 4√2. Thus, the radius of the circle is half of the diameter, or 2√2. The area of the circle is then π(2√2)2, which equals 8π. Next, the area of the square must be subtracted from the entire circle, yielding an area of 8π-16 square inches.

3

Circle_graph_area3

100_π_

50_π_

25_π_

10_π_

20_π_

Explanation

Circle_graph_area2

4

A square with a side length of 4 inches is inscribed in a circle, as shown below. What is the area of the unshaded region inside of the circle, in square inches?

Act_math_01

8π - 16

4π-4

8π-4

2π-4

8π-8

Explanation

Using the Pythagorean Theorem, the diameter of the circle (also the diagonal of the square) can be found to be 4√2. Thus, the radius of the circle is half of the diameter, or 2√2. The area of the circle is then π(2√2)2, which equals 8π. Next, the area of the square must be subtracted from the entire circle, yielding an area of 8π-16 square inches.

5

In a large field, a circle with an area of 144_π_ square meters is drawn out. Starting at the center of the circle, a groundskeeper mows in a straight line to the circle's edge. He then turns and mows ¼ of the way around the circle before turning again and mowing another straight line back to the center. What is the length, in meters, of the path the groundskeeper mowed?

24_π_

12 + 6_π_

12 + 36_π_

24 + 6_π_

24 + 36_π_

Explanation

Circles have an area of πr_2, where r is the radius. If this circle has an area of 144_π, then you can solve for the radius:

πr_2 = 144_π

r 2 = 144

r =12

When the groundskeeper goes from the center of the circle to the edge, he's creating a radius, which is 12 meters.

When he travels ¼ of the way around the circle, he's traveling ¼ of the circle's circumference. A circumference is 2_πr_. For this circle, that's 24_π_ meters. One-fourth of that is 6_π_ meters.

Finally, when he goes back to the center, he's creating another radius, which is 12 meters.

In all, that's 12 meters + 6_π_ meters + 12 meters, for a total of 24 + 6_π_ meters.

6

A circle with an area of 13_π_ in2 is centered at point C. What is the circumference of this circle?

2√13_π_

√13_π_

26_π_

√26_π_

13_π_

Explanation

The formula for the area of a circle is A = _πr_2.

We are given the area, and by substitution we know that 13_π_ = _πr_2.

We divide out the π and are left with 13 = _r_2.

We take the square root of r to find that r = √13.

We find the circumference of the circle with the formula C = 2_πr_.

We then plug in our values to find C = 2√13_π_.

7

In a large field, a circle with an area of 144_π_ square meters is drawn out. Starting at the center of the circle, a groundskeeper mows in a straight line to the circle's edge. He then turns and mows ¼ of the way around the circle before turning again and mowing another straight line back to the center. What is the length, in meters, of the path the groundskeeper mowed?

24_π_

12 + 6_π_

12 + 36_π_

24 + 6_π_

24 + 36_π_

Explanation

Circles have an area of πr_2, where r is the radius. If this circle has an area of 144_π, then you can solve for the radius:

πr_2 = 144_π

r 2 = 144

r =12

When the groundskeeper goes from the center of the circle to the edge, he's creating a radius, which is 12 meters.

When he travels ¼ of the way around the circle, he's traveling ¼ of the circle's circumference. A circumference is 2_πr_. For this circle, that's 24_π_ meters. One-fourth of that is 6_π_ meters.

Finally, when he goes back to the center, he's creating another radius, which is 12 meters.

In all, that's 12 meters + 6_π_ meters + 12 meters, for a total of 24 + 6_π_ meters.

8

A circle with an area of 13_π_ in2 is centered at point C. What is the circumference of this circle?

2√13_π_

√13_π_

26_π_

√26_π_

13_π_

Explanation

The formula for the area of a circle is A = _πr_2.

We are given the area, and by substitution we know that 13_π_ = _πr_2.

We divide out the π and are left with 13 = _r_2.

We take the square root of r to find that r = √13.

We find the circumference of the circle with the formula C = 2_πr_.

We then plug in our values to find C = 2√13_π_.

9

What is the area, in square feet, of a circle with a circumference of ?

Explanation

In order to find the area of a circle with a known circumference, first solve for the radius of the circle.

We know the circumference of a circle is equivalent to , where .

The radius of a circle is equal to half the diameter.

Therefore:

The area of a circle is given by the equation . Use the radius to solve for the area.

The area of a circle with circumference is square feet.

10

What is the area, in square feet, of a circle with a circumference of ?

Explanation

In order to find the area of a circle with a known circumference, first solve for the radius of the circle.

We know the circumference of a circle is equivalent to , where .

The radius of a circle is equal to half the diameter.

Therefore:

The area of a circle is given by the equation . Use the radius to solve for the area.

The area of a circle with circumference is square feet.

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