一个三角形的所有角度之和是180度。
只有在平坦的宇宙中，空间才符合标准的几何规律——三角形的内角和精确地等于180度，平行线永远不相交。
Only in a flat universe does space obey all the standard geometry-class rules-the angles of a triangle add up to exactly 180 degrees and parallel lines never meet.
你可以使用初等的几何知识例如三角形的三个内角各为180度和三角形全等的规则(边-角-边等)。
You may only use elementary geometry, such as the fact that the angles of a triangle add up to 180 degrees and the basic congruent triangle rules (side-angle-side, etc.).
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