A circle of radius r has a sector marked off by a central angle. Drag the angle handle around the circle to change the angle, or drag the rim handle to change the radius. The arc along the rim is highlighted and the pie-slice sector is shaded. The arc length is the fraction angle-over-360 of the whole circumference, and the sector area is the same fraction of the whole circle's area. The angle, that fraction, the arc length and the sector area are all shown live. An info button opens a drawer with the formulas.