Euclid, Elements of Geometry, Book I, Proposition 37
(Edited by Dionysius Lardner, 1855)

Proposition XXXVII. Theorem.
[Euclid, ed. Lardner, 1855, on Google Books]

(168) Triangles (B A C and B F C) on the same base and between the same parallels are equal.

Through the point B draw B E parallel to C A, and draw B D parallel to C F, and produce A F to meet these lines at E and D. A B C D E F The figures B E A C and B D F C are parallelograms on the same base B C and between the same parallels, and therefore, (XXXV) equal; and the triangles B A C and B F C are their halves (XXXIV), and therefore also equal.

Book I: Euclid, Book I (ed. Dionysius Lardner, 11th Edition, 1855)

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