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

## Example Questions

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### Example Question #26 : Radius

You are conducting fieldwork, when you find a tree whose radius at chest height is . What is the area of a cross section of the tree at chest height?

Explanation:

You are conducting fieldwork, when you find a tree whose radius at chest height is . What is the area of a cross section of the tree at chest height?

Begin with the formula for area of a circle

Now, we know r, so all we have to do is plug it in and solve.

### Example Question #28 : Radius

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

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 is 16in. We also know the diameter is two times the radius. Therefore, the radius is 8in. So, we get

### Example Question #27 : Radius

A circle has a diameter of 18cm. Find the area.

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 18cm.  We also know the diameter is two times the radius.  Therefore, the radius is 9cm.

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

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