I.5 Proposition I.5
In isosceles triangles the angles at the base are equal to one another; and if the equal straight lines be produced further, the angles under the base will be equal to one another.
Proof
Apply I.3 to mark equal segments on the produced sides, then I.4 to
two pairs of congruent triangles. Common Notion 3 gives equality of
the remaining angles.
lines 74–74 in main.tex
