Basic Geometry : Plane Geometry

Study concepts, example questions & explanations for Basic Geometry

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Example Questions

Example Question #106 : How To Find Circumference

Find the circumference  of a circle whose diameter, , is 10.

Note: 

Possible Answers:

Correct answer:

Explanation:

The formula for circumference is 

.

We were initially given diameter, so to get the radius we divide d by 2. 

.

We then solve by saying 

Example Question #302 : Circles

The area of a circle is . What is the circumference? 

Possible Answers:

Correct answer:

Explanation:

The area of a circle is , so in this case the radius squared is 4. Since the square root of 4 is 2, the radius has length 2. To find the circumference, use the formula . In this case, 

Example Question #301 : Circles

Circle radius

The radius of the circle shown here is  units long. What is its circumference?

Possible Answers:

Correct answer:

Explanation:

The formula for the circumference  of a circle given its radius  is . For the circle given in the problem,  units, and so

.

Hence, the circumference of this circle is  units.

Example Question #301 : Radius

A circle has diameter 20.

True or false: The circumference is greater than 60.

Possible Answers:

True

False

Correct answer:

True

Explanation:

The circumference of a circle is its diameter multiplied by , so, since the diameter is 20, the circumference is . Also, 

,

so, multiplying both sides by 20 by the Multiplication Property of Inequality, 

and

.

The circumference is indeed greater than 60.

Example Question #301 : Plane Geometry

Ashley has a square room in her apartment that measures 81 square feet. What is the circumference of the largest circular area rug that she can fit in the space?

Possible Answers:

Correct answer:

Explanation:

In order to solve this question, first calculate the length of each side of the room. 

The length of each side of the room is also equal to the length of the diameter of the largest circular rug that can fit in the room. Since , the circumference is simply

Example Question #1 : How To Find The Length Of The Diameter

Give the diameter of a circle whose circumference is .

Possible Answers:

Correct answer:

Explanation:

Example Question #1 : How To Find The Length Of The Diameter

The perimeter of a circle is 36 π.  What is the diameter of the circle?

Possible Answers:

3

6

36

18

72

Correct answer:

36

Explanation:

The perimeter of a circle = 2 πr = πd

Therefore d = 36

Example Question #302 : Plane Geometry

Two circles have only one point in common. Both circles have radii of . If point  is on the first circle and point  is on the second circle, what is longest possible distance of line ?

Possible Answers:

18

20

24

6

12

Correct answer:

24

Explanation:

Circle

The first step is to sketch two circles touching at a single point. In order to maximize the length of , the point and the point would need to be on opposite ends, as shown in the diagram. If the radius of a circle is , then the diameter would be . Therefore, the length of would be .

Example Question #1 : How To Find The Length Of The Diameter

Circle_line_identity

What is the name of the segment in green?

Possible Answers:

Radius

Ray

Diagonal

Chord

Diameter

Correct answer:

Diameter

Explanation:

The diameter is the maximum distance between two points on a circle's perimeter. The diameter passes through the circle's center.

Example Question #303 : Plane Geometry

The area of a circle is .  What is its diameter?

Possible Answers:

Correct answer:

Explanation:

First solve for the radius:

 

  

Note that , where is the radius and is the diameter. 

Therefore, .

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