📚 Classifying Triangles by Sides and Angles | 三角形的分类（按边与角）
Triangles are one of the most important shapes in geometry. In IGCSE mathematics, you need to classify triangles by their side lengths and by their interior angles. This skill helps you solve problems involving angle sums, inequalities, and even trigonometry later. Let us start from the basic angle facts and build up to complete triangle classification.
三角形是几何学中最重要的图形之一。在IGCSE数学中，你需要根据边长和内角大小对三角形进行分类。掌握这一技能有助于解决涉及内角和、三角形不等式甚至后续三角函数的问题。让我们从基础角知识出发，逐步建立完整的三角形分类体系。
1. Key Angle Types | 关键角类型
An angle is formed by two rays sharing a common endpoint. In geometry, angles are measured in degrees (°). Before classifying triangles, you must be able to identify acute, right, and obtuse angles instantly.
角由两条共端点的射线组成。在几何中，角度以度（°）为单位。在对三角形分类之前，你必须能迅速识别锐角、直角和钝角。
| Angle Type | 角类型 | Measure | 度数 | Visual Hint | 图形特征 |
|---|---|---|
| Acute | 锐角 | 0° < angle < 90° | Sharp point | 尖角 |
| Right
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