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# Applying pi

## Circumference

Using only this photo, could you find the circumference of this ferris wheel to within a few feet?

First we could take the height of a person who looks to be around 6 feet and figure out the radius of the circle:

5 * 6 = roughly 30 feet

With the radius we can use

**π**to determine the**circumference**directly:We correctly find that it’s approximately 188 feet around the perimeter of the wheel, all thanks to π!

## Area

How can we estimate the area of any circle given its radius?

Let’s approximate the answer using a pizza slice analogy.

**Below is an interactive illustration;**click the arrows to change the number of slices in the pie:As we increase n, the perimeter of the pizza approaches the circumference of the circle. As we increase the number of slices:

- the height of the slice approaches the radius of the circle (
**h = r**) - the area of the pizza approaches the area of the circle (
**area of pizza = area of circle**)

This leads to a well-known equation which relates the area and radius of any circle:

## Want to join the conversation?

- How do you find area when all you have is circumference (60cm)?(4 votes)
- Divide by pi to get the diameter, then divide that by two to get the radius, then solve the formula for the area of a circle. pi r squared.(4 votes)

- I understand pi x r squared, I understand how pi x r x r will get you the area, but the interactive illustration above shows area as 1/2 x circumference, and I'm not seeing that in the model. Can someone please explain?(3 votes)
- It's not showing area as 1/2 * circumference. It is showing area = base * height. The base is 1/2 * circumference or πr. The height is r. So, area = πr².(3 votes)

- Is there any other formulas for circle or ellipse?(4 votes)
- what is the number of pi?(2 votes)
- Here is a link to the first million digits: http://www.piday.org/million/(4 votes)

- What is circumference relative to perimeter?(1 vote)
- Perimeter = the distance around the outside (The definition my 3rd grade teacher gave) The circumference is the distance around the outside of a circle.(4 votes)

- what does pi mean and how did it get its simbol?(2 votes)
- How would u work this problem(1 vote)
- You divide the circumference of a circle by its diameter.

If you want a more approximate value for this, go to http://www.piday.org/million/(2 votes)

- sorry but this might be a stupid question.....what does circumference mean? And how does pi=circumference/diameter help you find the circumference directly?(0 votes)
- What formula do you use to work this out?(0 votes)
- how do we find out the relation Circumference=2*pi*r(0 votes)