It is also constructible with compass, straightedge and angle trisector. Classic geometry was focused in compass and straightedge constructions. Angle trisection is a classical problem of compass and straightedge constructions of ancient Greek mathematics. A square root can be constructed with a compass and straightedge. No continuous arc of a conic can be constructed with straightedge and compass. The regular pentagon is constructible with compass and straightedge, as 5 is a Fermat prime. A number representing a length that can be constructed using a compass and straightedge. Some regular polygons are easy to construct with compass and straightedge; others are not. The following animations show how to construct a square using a compass and straightedge. An approximate construction of a regular tridecagon using straightedge and compass is shown here. Straight lines are drawn with a pencil and an unmarked straightedge. A ruler can be used both as a straightedge and a device to compute proportions. The mathematical theory of origami is more powerful than straightedge and compass construction. Another possible animation of an approximate construction, also possible with using straightedge and compass. It is also feasible with exclusive use of compass and straightedge.