📚 Mensuration: Area, Perimeter & Volume | 度量：面积、周长与体积
Mensuration is the study of measuring lengths, areas and volumes of shapes and solids. In IGCSE Mathematics, it is one of the most heavily tested topics, appearing in both Paper 2 (short-answer questions) and Paper 4 (extended problem-solving). A strong grasp of mensuration also supports other topics such as trigonometry, algebra and ratio. This article covers every formula you need, explains common pitfalls, and provides worked examples written in the style of IGCSE mark schemes.
度量是研究图形与立体之长度、面积和体积测量的数学分支。在 IGCSE 数学中，它是考查频率最高的主题之一，Paper 2（简答题）与 Paper 4（拓展应用题）都会涉及。扎实掌握度量还能为三角函数、代数与比例等其他主题提供支撑。本文涵盖所有必备公式、常见陷阱，并给出完全参照 IGCSE 评分标准风格的例题解答。
1. Perimeter and Area of Basic 2D Shapes | 周长与基本平面图形的面积
Every 2D shape has a perimeter (the total distance around its boundary) and an area (the amount of space inside it). The following formulas must be recalled without a calculator and written down correctly before substituting values.
每个平面图形都有周长（边界总长度）与面积（内部空间大小）。以下公式必须能脱离计算器直接回忆，并在代入数值前正确写出。
| Shape 图形 | Perimeter 周长 | Area 面积 |
| Rectangle 矩形 | P = 2(l + w) | A = l × w |
| Square 正方形 | P = 4s | A = s² |
| Triangle 三角形 | P = a + b + c | A = ½ × base × height |
| Parallelogram 平行四边形 | P = 2(a + b) | A = base × height |
| Trapezium 梯形 | P = a + b + c + d | A = ½ × (a + b) × h |
| Circle 圆 | C = 2πr | A = π
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