T H E E L E M E N T S |
Book VI Proposition 2 | |
Therefore each of the triangles BDE, CDE has the same ratio to ADE. Therefore the triangle BDE is equal to the triangle CDE; [V.9] and they are on the same base. But equal triangles which are on the same base are also in the same parallels. [I.39] Therefore DE is parallel to BC. |
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