# ISEE Upper Level Math : How to find the area of a circle

## Example Questions

### Example Question #52 : Geometry

What is the area of a circle with a diameter of , rounded to the nearest whole number?

Explanation:

The formula for the area of a circle is

Find the radius by dividing 9 by 2:

So the formula for area would now be:

### Example Question #44 : Circles

A circle has a radius of 5 miles, what is its area?

Explanation:

A circle has a radius of 5 miles, what is its area?

Find the area of a circle with the following formula:

We know that r is 5, so we can find our answer with the following:

### Example Question #45 : Circles

While mailing some very important letters, you decide to use your circular rubber stamp. If the stamp has a radius of 3.5 cm, what is the area of the stamping surface?

Explanation:

While mailing some very important letters, you decide to use your circular rubber stamp. If the stamp has a radius of 3.5 cm, what is the area of the stamping surface?

Find the area of a circle by using the following formula:

Our radius is 3.5 cm, so plug that in:

### Example Question #46 : Circles

You have a circular window in your vacation room. It has a radius of 9 inches. What is the area of the window?

Explanation:

You have a circular window in your vacation room. It has a radius of 9 inches. What is the area of the window?

To find the area of a circle, use the following formula:

Now, we know the radius, so we just need to plug it in and solve.

### Example Question #21 : Radius

Find the area of a circle with a radius of 6in.

Explanation:

To find the area of a circle, we will use the following formula:

Now, we know the radius of the circle is 6in.

Knowing this, we can substitute into the formula.  We get

### Example Question #22 : Radius

Find the area of a circle with a diameter of 10in.

Explanation:

To find the area of a circle, we will use the following formula:

where is the radius of the circle.

Now, we know the diameter of the circle is 10in.  We also know the diameter is two times the radius.  Therefore, the radius is 5in.

Knowing this, we can substitute into the formula.  We get

### Example Question #23 : Radius

Find the area of a circle with a diameter of 14cm.

Explanation:

To find the area of a circle, we will use the following formula:

where r is the radius of the circle.

Now, we know the diameter of the circle is 14cm.  We also know the diameter is two times the radius.  Therefore, the radius is 7cm.

Knowing this, we can substitute into the formula.  We get

### Example Question #24 : Radius

You have a tomato plant growing in a pot. If the base of the pot has a radius of 6 inches, find the area of the footprint of the pot.

Explanation:

You have a tomato plant growing in a pot. If the base of the pot has a radius of 6 inches, find the area of the footprint of the pot.

To solve this problem, first recall the formula for area of a circle.

Now, we know our r=6 inches, so we just need to plug in and simplify.

### Example Question #11 : How To Find The Area Of A Circle

Let .

Find the area of a circle with a diameter of 14cm.  If necessary, round to the nearest tenths.

Explanation:

To find the area of a circle, we will use the following formula:

where r is the radius of the circle.

Now, we know the diameter of the circle is 14cm.  We also know that the diameter is two times the radius.  Therefore, the radius is 7cm.

We also know .

Knowing all of this, we can substitute into the formula.  We get

We were asked to round to the nearest tenth.  So, we get

### Example Question #25 : Radius

Find the area of a circle with a radius of 16in.