Euclid I-23. (pg 18)

Emily Crum
Explanation: Let there exist line AB and angle CDE. Let C and E be chosen be taken at random and CE joined. Pick pt F on AB such that AF =DE. let AG= CD and FG= CE. (Using circle radii and the proposition before this one in the book) By SSS, these triangles are congruent. Thus, Angle FAG = angle DCE. Therefore, a rectilineal angle on a line had been constructed such that it is = to another rectilineal angle.