Corollary: Triangulation of Quadrilateral and Sum of Angles
(related to Proposition: 1.32: Sum Of Angles in a Triangle and Exterior Angle)
Since a quadrilateral can be divided into two triangles, the sum of its angles equals four right angles.
\[\sum_{i=1}^6 \alpha_i=2\times 180^\circ=4\times 90^\circ\]
Table of Contents
Proofs: 1
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References
Adapted from CC BYSA 3.0 Sources:
 Callahan, Daniel: "Euclidâ€™s 'Elements' Redux" 2014
Adapted from (Public Domain)
 Casey, John: "The First Six Books of the Elements of Euclid"