The Circumference and Area of a Circle

Drag the Roll circle slider, or click the triangular "play" button in the lower left to measure the circumference of the circle along the horizontal axis.

The "Show C..." checkbox shows the same length along the axis measured with diameters of the circle.

To see the axis marked in different units (circumference, diameter, radius) click the next check boxes.

The check boxes to the right show the steps in dissecting a circle to a parallelogram or rectangle.

The last check box shows how to see the area of a circle: as a bit more than 3 squares, where the square is made from the radius of the circle.
This explains the formula: pi r squared.

Formula:

Susan Addington, Created with GeoGebra

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