A circle is not a polygon because ... has sides that are equal in length and equal angles. These shapes are all regular polygons. A regular triangle is called an equilateral triangle.

If you are using a ruler and compass to draw something - for example, a perpendicular bisector ... the height of the triangle is known, trigonometry can be used to find the angle marked x.

This example was written by Sam Britt answering a question on TeX.SE.

Figure 1.1: Parameters of a "circle" roundover Figure 1.2 ... The "pointed" end treatment caps the stroke with a centered triangle that has 45 degree angles on each side. More complex end treatments ...