Euclid Book 1 - Proposition 14
Angles adding up to 180° make a straight line

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To be proved: If with any straight line, and at a point on it, two straight lines not lying on the same side make the sum of the adjacent angles equal to two right angles, then the two straight lines are in a straight line with one another.

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There is a straight line AB with two lines either side CB and DB, where angles CBA and ABD add up to 180°

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© Jo Edkins 2010 - Return to Euclid index