Geometry for elementary school/Copying an angle

Introduction

In this chapter, we will show how to copy an angle $\mathrm{\angle }ABC$ to other angle $\mathrm{\angle }CDE$ . The construction is based on Book I, proposition 23.

The construction

1. Draw a line between A and B and get $\mathrm{△}ABC$.
2. Copy the triangle $\mathrm{△}ABC$ and get $\mathrm{△}CDE$

Claim

The angles $\mathrm{\angle }ABC$ and $\mathrm{\angle }CDE$ are equal.

Proof

1. The triangles $\mathrm{△}ABC$ and $\mathrm{△}CDE$ congruence.
2. Therefore the angles of the triangles equal.
3. Hence, $\mathrm{\angle }ABC$ and $\mathrm{\angle }CDE$ are equal.

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