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Euclid's Elements, Book I, Proposition 32

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I say that the exterior angle ACD equals the sum of the two interior and opposite angles CAB and ABC, and the sum of the three interior angles of the triangle ABC, BCA, and CAB equals two right angles.

Euclid's Elements, Book I, Proposition 32 Proposition 32 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. Let ABC be a triangle, and let one side of it BC be produced to D. I say that the exterior angle ACD equals the sum of the two interior and opposite angles CAB and ABC, and the sum of the three interior angles of the triangle ABC, BCA, and CAB equals two right angles. I.31 Draw CE through the point C parallel to the straight line

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