Proposition XXX. Theorem.
|(121)||If two right lines (A B, C D) be parallel to the same right line (E F), they are parallel to each other.|
Let the right line G K intersect them; the angle A G H is equal to the angle G H F (XXIX); and also the angle H K D is equal to G H F (XXIX); therefore A G H is equal to G K D; and therefore the right lines A B and C D are parallel.
Book I: Euclid, Book I (ed. Dionysius Lardner, 11th Edition, 1855)
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