Congruent Triangles by Equal Base and Angles
Equal base cb = fe and equal adjacent angles make all sides and angles equal
The folio proves the equality of two triangles, lettered a b c and d e f: because base "c b is equal to side f e" and the adjacent angles c and f, and b and e, are equal, "the other sides and angles are equal." Curved pen-strokes tie base c b to base f e, and a dot marks the corresponding lower-left angle of each triangle. Below, a triangle f d e is cut by a line from a vertex to point g on the opposite side, with a further similar triangle sketched alongside. The leaf carries the number 27 and an encircled 261.
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Equality of two triangles from base and angles
Two triangles are lettered a b c and d e f, their bases c b and f e joined by curved strokes. The argument runs that c b equals side f e, and since angles c and f, and b and e, are equal, therefore the other sides and angles are equal.
Triangle f d e divided to point g
A triangle lettered f, d, e is divided by a line running from one vertex to the point g on the opposite side, extending the comparison of triangles below the main proof.