Arcs are fundamental objects in Euclidean geometry. In Sketchpad, use the Construct menu to create arcs. An arc is a path object. You can change the object properties, label properties, and display attributes of arcs. 
If you construct an arc through three points and then drag the three points so they are collinear, the second point determines the appearance of the arc. If the second point falls between the first and third poins, the arc has zero angle measure but nonzero arc length; it is displayed as a segment that starts at the first selected point, passes through the second point, and ends at the third point. If the second point is collinear with, but not between, the first and third points, the arc is not well defined: it disappears and any measurements that depend on it are undefined. 
Sketchpad provides two different ways to construct an arc.

•Construct a point on an arc using the Point tool or the Construct  Point on Object command. Then animate the point along the arc or use the point as the driver for a locus. •Attach an independent point to the arc by using the Edit  Merge command.
•Use Construct  Intersection to construct an arc’s intersection with a straight object, a circle, another arc, a point locus, or a function plot. 
Use the Measure menu to find: •the Radius of an arc •the Arc Length of an arc •the Arc Angle of an arc 