# Common Core: 7th Grade Math : Circumference of a circle

## Example Questions

### Example Question #2 : Perimeter

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

Explanation:

The circumference can be solved using the following equation:

### Example Question #1 : Circumference Of A Circle

The radius of a circle is . Give the circumference of the circle in terms of .

Explanation:

The circumference can be calculated as , where is the radius of the circle.

### Example Question #1 : Circumference Of A Circle

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

Explanation:

The circumference can be solved using the following equation:

Where  represents the radius. Therefore, when we substitute our radius in we get:

### Example Question #1 : Circumference Of A Circle

If this circle has a diameter of 12 inches, what is its circumference?

none of these

Explanation:

Know that the formula for circumference is , where C is the circumference and D is the diameter.  It is given that the diameter is 12 inches.  Therefore, the circumference is

### Example Question #1 : How To Find Circumference

If a circle has an area of , what is the circumference of the circle?

Explanation:

The formula for  the area of a circle is πr2. For this particular circle, the area is 81π, so 81π = πr2. Divide both sides by π and we are left with r2=81. Take the square root of both sides to find r=9. The formula for the circumference of the circle is 2πr = 2π(9) = 18π. The correct answer is 18π.

### Example Question #1 : Circumference Of A Circle

If a circle has an area of , what is the circumference?

Explanation:

For a circle, the formula for area is  and the formula for circumference is , where  is the radius and  is the diameter.

Plug the known quantities into the area formula and solve for the radius:

Now plug this value into the circumference formula to solve:

### Example Question #1 : Circumference Of A Circle

Find the circumference of a circle with a radius of .

Explanation:

Recall the formula for finding the circumference of a circle:

We can substitute in the value for the radius in order to find the circumference of the circle in question.

Solve.

Simplify.

### Example Question #2 : Circumference Of A Circle

Find the circumference of a circle that has a radius of .

Explanation:

Recall the formula for finding the circumference of a circle:

We can substitute in the value for the radius in order to find the circumference of the circle in question.

Solve.

Simplify.

### Example Question #1 : Circumference Of A Circle

Find the circumference of the circle with a radius of .

Explanation:

Recall the formula for finding the circumference of a circle:

We can substitute in the value for the radius in order to find the circumference of the circle in question.

Solve.

Simplify.

### Example Question #1 : Circumference Of A Circle

Find the circumference of a circle with a radius of .