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the angles BAC, ABC, ACB are equal to two right angles; [I.32]
therefore
the angles ACE, ACB are also equal to two right angles.
Therefore
With a straight line AC, and at the point C on it, the two straight lines BC, CE, not lying on the same side make the adjacent angles ACE, ACB equal to two right angles;
therefore
BC is in a straight line with CE. [I.14]
