Entry Nº 022Stage 2 · Grades 3–4图形与几何
射线与直线
线段一端延长得射线，两端延长得直线。
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Definition
把线段向一端无限延长，得到射线（一个端点）；向两端无限延长，得到直线（没有端点、不可度量）。手电筒的光束近似射线。
Where it comes from · where it goes
Prerequisites
What builds on it