If a parallelogram have the same base with a triangle and be in the same parallels, the parallelogram is double of the triangle.
若平行四边形与三角形在同一底边、同两条平行线之间，则平行四边形是该三角形的二倍。
平行四边形 ABCD 与三角形 BEC 共底 BC、夹在同两条平行线之间；则平行四边形面积是三角形的两倍。
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For let the parallelogram ABCD have the same base BC with the triangle EBC, and let it be in the same parallels BC, AE; I say that the parallelogram ABCD is double of the triangle BEC. For let AC be joined.
平行四边形与三角形同底、同两条平行线之间。
Then the triangle ABC is equal to the triangle EBC; for it is on the same base BC with it and in the same parallels BC, AE.
作平行四边形的对角线，它把平行四边形分成两个相等三角形（euclid-elements/book1-prop-034）。
[I. 37] But the parallelogram ABCD is double of the triangle ABC; for the diameter AC bisects it; [I. 34] so that the parallelogram ABCD is also double of the triangle EBC.
其中一个三角形与给定三角形同底同平行线间，由 euclid-elements/book1-prop-037 相等。
Therefore etc.
所以整个平行四边形是给定三角形的二倍。