Recall the worksheet on and radians (http://www.geogebratube.org/material/show/id/29578). In that worksheet, is defined to be the ratio of a circle's circumference (C) to its diameter (d). That is, =C/d. Therefore, C=d. Since d=2r, you can see that C=2r. The formula for the area of a circle is slightly more difficult do derive. Here are two methods of doing so.