Euclid Book 1 - Proposition 32
Angles of a triangle add up to 180°

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To be proved: In any triangle, if one of the sides is produced, then the exterior angle equals the sum of the two interior and opposite angles, and the sum of the three interior angles of the triangle equals two right angles.

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There is a triangle ABC.

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© Jo Edkins 2010 - Return to Euclid index