Area of a Circle

After finding the formula for the circumference of a circle, my 6th graders were ready to work on finding the area of a circle.

I asked them to draw a circle on notebook paper, any size, but not too small. Then I gave each a centimeter cube to trace one face onto their paper to remind them how much the area of one square centimeter covered. (You can see it drawn on top right pic below.)

Question 1: Give me a guess — only by looking — what is the area of your circle, in square centimeters? Please write that number on your paper, label it “guess.”

Question 2: Now use whatever tools you need, give me a better answer. I know some of you already know the formula for the area of a circle, but you may not use it unless you can tell me where it came from. (No one even tried.)

I can’t tell you how happy I was to see all the different ways the the kids had tried to approximate area. Their perseverance humbled me. A few students drew triangles, knew to use “base times height divided by two,” but erroneously used radius as height.

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Last week I was reading Mimi’s post about estimating area of circles, and @suevanhattum reminded me of the rectangle model in her comment.

I had the kids fold their circles like this.

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They cut out the pieces — turning every other piece 180 degrees — and glued them together. Cristian said, “First we had a circle, then triangles, then a rectangle. That’s crazy!”

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Never once did I answer any of the questions. I just asked them. I began with, “What is the area of this rectangle or parallelogram?” Step by step, as a class, the kids walked this equation all the way to Area = pi x radius^2. :-)

This morning, two days after the activity, I did a My Favorite No to see if they remembered: 31 out of 33 students got the correct circumference formula; 24 of 33 got the correct area formula. Bonus if they showed me how the rectangle model helped explain the area of a circle.

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One Comment

  1. Posted April 23, 2015 at 8:58 am | Permalink

    I’ve tried teaching this by cutting up the sectors of a circle to make rectangle. Works really well!

2 Trackbacks

  • By Geometry Class | Ms Moore Math on January 14, 2015 at 11:10 am

    […] Area of Circle using centimeter cubes […]

  • By 7 Deadly Sins of Teaching [Maths] on August 24, 2016 at 10:54 am

    […] Giving out the equation. That’s like giving away life’s secrets to someone who flies to Paris to have lunch. Meaning, they don’t need it, nor did they ask for it. Your students’ conversations, their conjectures, their models — are all at the heart of a math class. To give away the equation is to passively (and aggressively!) dismiss our students’ abilities to think for themselves. It’s okay to eventually give them the equation in due time, just don’t start with the equation. Imagine if I just gave my students the equations for slope and area of a circle. […]

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