Circles are fundamental objects in Euclidean geometry. In classical geometric constructions, you use a compass to construct circles. A circle is a path object. You can change the object properties, label properties, and display attributes of circles. 
Sketchpad provides several different ways to construct a circle.

•Construct a point on a circle using the Point tool or the Construct  Point on Object command. Then animate the point around the circle or use the point as the driver for a locus. •Attach an independent point to the circle by using the Edit  Merge command. •Use Construct  Interior to construct the interior of a circle. •Use Construct  Intersection to construct a circle’s intersection with a straight object, another circle, an arc, a point locus, or a function plot. •Use Graph  Define Unit Circle to define a coordinate system that uses the selected circle as its unit circle. 
Use the Measure menu to find: •the Circumference of a circle •the Radius of a circle •the Area of a circle •the Equation of a circle 