This is one of the facts that characterizes [[Euclidean geometry]]: it is not, however, true in other geometries. The proof for this theorem is reliant on the parallel postulate, so does not generalize into [[Non-Euclidean geometry|other geometries]]. For example, any triangle constructed on the surface of the earth (known as a spherical triangle) the sum of the angles is always greater than <math> 180^o </math>. | This is one of the facts that characterizes [[Euclidean geometry]]: it is not, however, true in other geometries. The proof for this theorem is reliant on the parallel postulate, so does not generalize into [[Non-Euclidean geometry|other geometries]]. For example, any triangle constructed on the surface of the earth (known as a spherical triangle) the sum of the angles is always greater than <math> 180^o </math>. |