# GED Math : Radius and Diameter

## Example Questions

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### Example Question #1 : Circles

What is the radius of a circle with a diameter of 12 inches?

Explanation:

The radius is half of the diameter:

From here we plug in our diameter measure of 12 and solve for the radius:

### Example Question #2 : Circles

What is the diameter of a circle with radius of 10 inches?

Explanation:

The diameter is twice the radius:

### Example Question #1 : Radius And Diameter

What is the radius of a circle that has a circumference of ?

20

12

4

18

18

Explanation:

The equation for circumference of a circle is as follows:

Plug in the given values and solve for the radius:

Now we divide each side by :

### Example Question #4 : Circles

Find the diameter of a circle that has a circumference of ?

25

12.5

20

10

25

Explanation:

We know the equation for the circumference of a circle is . We also know that the the diameter is the twice the radius, so we can write .

Plug in the given values and solve:

### Example Question #5 : Circles

What is the diameter of a circle with a radius of 9?

16

6

20

18

18

Explanation:

The diameter of a circle is twice the radius:

### Example Question #2 : 2 Dimensional Geometry

What is the radius of a circle given the diameter is 22?

11

6

9

15

11

Explanation:

The radius is half of the diameter, or 11.

### Example Question #2 : Radius And Diameter

What is the diameter of a circle with an area of ?

Explanation:

In order to solve this question, start by finding the radius.  Recall that the area of a circle is computed as:

For our data, this is:

Solving for , we get:

or

Now, recall that the diameter is double the radius.  Thus, it is .

### Example Question #3 : Radius And Diameter

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

Cannot be computed from the information provided

Explanation:

Recall that the circumference formula is:

Now, for your data, this gives you:

You need to be careful, for some students get confused when they have to divide by .  However, just treat it like any other number or variable.  This means that you can solve for  and get:

### Example Question #4 : Radius And Diameter

What is the diameter of a circle given the area of 30?

Explanation:

Write the area formula of a circle.

Substitute the area.

Divide by pi on both sides.

Square root both sides.

The diameter is double the radius.

### Example Question #5 : Radius And Diameter

What is the diameter of a circle if the area is ?

Explanation:

Write the formula for the area of a circle.

Substitute the area into the equation.

Divide by pi on both sides.

The diameter is twice the radius.