Geometry for Elementary School/Copying an angle

Geometry for Elementary School
Copying a triangle Copying an angle Constructing a triangle

In this chapter, we will show how to copy an angle to other angle . The construction is based on Book I, proposition 23.

The construction

  1. Draw a line between A and C and get  .
  2. Copy the triangle   and get  



The angles   and   are equal.


  1. The triangles   and   congruence.
  2. Therefore the angles of the triangles equal.
  3. Hence,   and   are equal.

Note that any two points on the rays can be used to create a triangle.