1. A 120 degree angle has an interior angle of 120 and an exterior angle of 60. 2. A 150 degree angle has an interior angle of 150 and an exterior angle of 30. 3. The exterior angle theorem is not valid in spherical geometry nor in the related elliptical geometry. 4. One interior angle in a regular triacontagon is 168? meaning that one exterior angle would be 12? 5. In other words, 360 " k " ?represents the sum of all the exterior angles . 6. Equivalently, a convex quadrilateral is cyclic if and only if each exterior angle is equal to the opposite interior angle. 7. This contradicts Proposition 16 which states that an exterior angle of a triangle is always greater than the opposite interior angles. 8. However, since the Euclid's exterior angle theorem is a theorem in absolute geometry it is automatically valid in hyperbolic geometry. 9. The method can be extended to any regular polygon by truncating the catenary at a point corresponding to the exterior angle ." 10. Exterior angles are quoined with white stone and / white stone " dripstone course " separates the lower and upper half of the building.
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