# GMAT Math : Circles

## Example Questions

### Example Question #7 : Calculating Circumference

If the radius of a circle is , what is its circumference?

Explanation:

Using the formula for the circumference of a circle, we can plug in the given value for the radius and calculate our solution:

### Example Question #81 : Geometry

Megan, a civil engineer, is designing a roundabout for the city of Madison. She knows that the distance from the edge of the roundabout to the center must be 25 meters. Help Megan find the circumference of the roundabout.

Explanation:

Megan, a civil engineer, is designing a roundabout for the city of Madison. She knows that the distance from the edge of the roundabout to the center must be 25 meters. Help Megan find the circumference of the roundabout.

We are asked to find circumference. In order to do so, look at the following formula:

Where r is our radius and C is our circumference.

We are indirectly told that our radius is 25 meters, plug it in to get our answer:

### Example Question #9 : Calculating Circumference

A circle has radius . Give its circumference.

Explanation:

The circumference of a circle is found using the following formula:

Set :

### Example Question #1 : Calculating The Length Of The Diameter

A square is inscribed inside a circle.  The square has an area of 100.  Find the area of the circle.

Explanation:

Since the square is inscribed inside the circle, then the length of the diagonal of the square will be the diameter of the circle.  The area of a circle is given as  and

Substituting the diameter into the equation of the area we get

So if we find the diagonal of the square, then we can find the area of the circle.  To find the diagonal of the square we use the fact that the area of a square is  where  is the side of the square.  Since the area is 100, then the length of the square is 10.  The diagonal is then given by

Substituting this into our equation for the area of a circle, we get

### Example Question #1 : Calculating The Length Of The Diameter

Find the center of the circle that has a diameter with endpoints at and .

Explanation:

### Example Question #1 : Calculating The Length Of The Diameter

Consider the Circle :

(Figure not drawn to scale.)

If  is the center of the circle and  is a point on its circumference, what is the length of the diameter of the circle?

Explanation:

This problem provides you with a circle and gives you clues that line  is the radius. A line that passes through the center and ends on the circumference of the circle is the radius of the circle. In this case, our radius is 15 meters. Diameter is simply twice the radius, so our diameter is 30 meters.

### Example Question #1 : Calculating The Length Of The Diameter

A circle has a circumference of . What is the diameter of the circle?

Explanation:

We can find the diameter of the circle using the formula for circumference:

First, we must recognize that the radius is just half of the diameter, which allows us to express the formula in terms of diameter:

Now we can just plug in the given circumference and solve for the diameter:

### Example Question #5 : Calculating The Length Of The Diameter

If the area of a circle is , what is the diameter of the circle?

Explanation:

We are given the area of the circle, so we will need to use the formula for area to calculate its diameter:

First, we must express the formula in terms of diameter instead of radius. The radius is just half of the diameter, so we can write:

Now we can simply plug in the given area and solve for the diameter:

### Example Question #1 : Diameter

A given circle has an area of . What is the length of its diameter?

Not enough information provided

Explanation:

The area  of a circle is defined by the equation , where  is the length of the circle's radius. The radius, in turn, is defined by the equation , where  is the length of the circle's diameter.

Given , we can deduce that  and therefore . Then, since .

### Example Question #321 : Gmat Quantitative Reasoning

A given circle has a radius of . What is the diameter of the circle?