Isosceles Triangle by the Common Notion of Equals
Lines equal to the same third line are equal to one another, applied to a two-equal-sided triangle
This all-text page reasons in Euclidean fashion that things equal to a third thing are equal to one another, concluding that lines ac and bc, both equal to a given line ab, are equal to each other. A second passage explains how to make a triangle with two equal sides, invoking the definition of the circle — all radii equal — and the postulate about equals to prove segments af and lb equal. A closing line notes that the figure below follows the same reasoning.
On this page
Things equal to a third are equal to one another
By the common notion that things equal to a third are equal among themselves, lines ac and bc, both equal to the given line ab, are shown to be equal to one another.
Constructing a triangle with two equal sides
To build a triangle with two equal sides, the definition of the circle gives bf equal to ba and ab equal to al; then, by the postulate on equals, la and bf, both equal to ab, are equal to one another, so af and lb are equal.
The figure below follows
A brief closing note states that the figure drawn below the text follows from the same reasoning.