Euclid, Elements of Geometry, Book I, Proposition 41
(Edited by Dionysius Lardner, 1855)

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

(190) If a parallelogram (B D) and a triangle (B E C) have the same base and be between the same parallels, the parallelogram is double of the triangle.

A B C D E Draw C A. The triangle B E C is equal to the triangle B A C (XXXVII); but B D is double of the triangle B A C (XXXIV), therefore B D is also double of the triangle B E C.

(191)   This proposition may be generalized thus: If a parallelogram and triangle have equal bases and altitudes, the parallelogram is double the triangle (175).

(192)   Also, If a parallelogram and a triangle have equal altitudes, and the base of the triangle be double the base of the parallelogram, the parallelogram and triangle will be equal (178).

(193)   If a parallelogram and triangle have equal bases, and the altitude of the triangle be double the altitude of the parallelogram, they will be equal.


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

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