Euclid Book I PROP. XXIX. -- THEOREM.

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Geometry

If a right line (EF) intersect two parallel right lines (AB, CD), it makes:

  1. the alternate angles (AGH, GHD) equal to one another;
  2. the exterior angle (EGB) equal to the corresponding interior angle (GHD);
  3. the two interior angles (BGH, GHD) on the same side equal to two right angles.

Euclid