Euclid's Elements — Book I, Proposition 32

In any triangle, if one of the sides be produced, the exterior angle is equal to the two interior and opposite angles, and the three interior angles of the triangle are equal to two right angles. Required Propositions: I.13 -    • Euclid's Elements — Book I, Proposition 13   I.29 -    • Euclid's Elements — Book I, Proposition 29   I.31 -    • Euclid's Elements — Book I, Proposition 31   Proof that the internal angles of a triangle sum to 180 degrees.