I.16 Proposition I.16
In any triangle, if one of the sides be produced, the exterior angle is greater than either of the interior and opposite angles.
Proof
Bisect a side by I.10; produce a median; apply I.4 to obtain a
congruent triangle that has the interior angle as one of its parts;
Common Notion 5 closes the inequality.
lines 74–74 in main.tex