Using the area of a circle formula for a little algebra review

My younger son and I started talking about circles this morning. Even before the first example problem in our Prealgebra book there’s a sketch of the proof that the area of a circle is \pi r^2. Seeing that proof, I closed the book and decided to walk through it with my son for today’s project.

To start I asked him what he already knew about circles – turns out that he’s heard a lot about circles. He even thinks to find the area of a circle by chopping it up into pieces:

In the next part we got a little sidetracked by some algebra. That’s fine. Instead of putting all of the focus on the geometry proof, I decided to take a little detour to try to make the algebra a bit more clear. I suspect that my son’s algebra mistakes here are pretty common mistakes. Hopefully we’ll get through some of these difficulties with a bit more practice.

Now back to the geometry. We chop up the circle into 16 pieces now and try to rearrange the shapes into a (near) rectangle. Our drawings probably aren’t exactly right, but at least you get the idea that the shape is starting to look more and more like a rectangle. With the extra algebra practice from the last video, the formula for the area of a circle now falls into place.

Maybe the next project is comparing the famous “proof” that \pi = 4 and seeing if he can see where things are going wrong. For today, though, a nice little proof and hopefully some valuable algebra practice. Fun morning.



One Comment so far. Leave a comment below.
  1. Ad,

    When you started talking about quesadillas, I thought you were going to arrange your lunch into a rectangle. I think I will pose that challenge to my child during lunch tomorrow. Tasty math!

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