# Circumference and Area of a Circle

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.[br][br]Check the "Show rolling circle" box, then roll the circle with the slider labeled "Roll circle".[br]Check "Show C measured with r, d."[br][br]This leads to a formula for the area of a circle, too. Cut it apart into sectors, and rearrange into a "parallelogram".[br][br]Check the last three boxes to see this.
Formulas:[br][br]Circumference = $\pi$ diameter (that is, $C=\pi d$)[br]Circumference = 2$\cdot\pi\cdot$radius (that is, $C=2\pi r$)[br][br]Area = Circumference $\cdot$ radius [br]Area = $\pi\cdot$ radius squared (that is, $A=\pi r^2$)