Exploration: Circumference and Area of a Circle APS
This applet illustrates formulas for the circumference by rolling a circle along a line, then measuring the length in units of the diameter, or the radius. Check the "Show rolling circle" box, then roll the circle with the slider labeled "Roll circle". Check "Show C measured with r, d." This leads to a formula for the area of a circle, too. Cut it apart into sectors, and rearrange into a "parallelogram". Check the last three boxes to see this.
Formulas: Circumference = diameter (that is, ) Circumference = 2radius (that is, ) Area = Circumference radius Area = radius squared (that is, )