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Angle trisection is the construction, using only a straightedge and a compass, of an angle that is one-third of a given arbitrary angle. This is impossible in the general case. For example, the angle 2 π /5 radians (72° = 360°/5) can be trisected, but the angle of π /3 radians (60°) cannot be trisected. [8]
The angle π / 3 radians (60 degrees, written 60°) is constructible. The argument below shows that it is impossible to construct a 20° angle. This implies that a 60° angle cannot be trisected, and thus that an arbitrary angle cannot be trisected. Denote the set of rational numbers by Q.
A circumferentor, or surveyor's compass, is an instrument used in surveying to measure horizontal angles. It was superseded by the theodolite in the early 19th century. [1] A circumferentor consists of a circular brass box containing a magnetic needle, which moves freely over a brass circle, or compass divided into 360 degrees.
Compass and straightedge constructions are known for all known constructible polygons. If n = pq with p = 2 or p and q coprime, an n-gon can be constructed from a p-gon and a q-gon. If p = 2, draw a q-gon and bisect one of its central angles. From this, a 2q-gon can be constructed.
A set square is used in technical drawing, providing a straightedge at a right angle or another particular planar angle to a baseline. They are commonly made from clear plastic. The most common set squares are 45° squares, (one 90° corner and two 45° corners) and 60/30 triangles (a 90°, a 60° and a 30° corner).
These set squares come in two usual forms, both right triangles: one with 90-45-45 degree angles, the other with 30-60-90 degree angles. Combining the two forms by placing the hypotenuses together will also yield 15° and 75° angles. They are often purchased in packs with protractors and compasses.
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To draw an angle, compasses describe an arc from origin with a radius taken from the 60 mark. The required angle is copied from the scale by the compasses, and an arc of this radius drawn from the sixty mark so it intersects the first arc. The line drawn from this point to the origin will be at the target angle. [1]